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porcupine: Biological Overview | References

Wnt signaling pathways regulate many developmental responses; however, little is known about how Wnt ligands function on a biochemical level. Recent studies have shown that Wnt-3a is palmitoylated before secretion. This study reports that Drosophila Wingless also undergoes a lipid modification. Lipidation occurs in the endoplasmic reticulum and is dependent on Porcupine, a putative O-acyltransferase. After modification, Wingless partitions as a membrane-anchored protein and is sorted into lipid raft detergent-insoluble microdomains. Lipidation, raft targeting, and secretion can be blocked by the addition of 2-bromopalmitate, a competitive inhibitor of O-acyltransferase activity. Based on these results a model is proposed whereby lipidation targets Wingless to secretory vesicles that deliver the ligand to specialized microdomains at the cell surface where it can be packaged for secretion (Zhai, 2004).

Wnt ligands play key roles in many developmental pathways. To understand how Wnts regulate so many developmental activities, it is necessary to define the biochemical steps that constitute the actual signal transduction events. With this goal in mind, the synthesis and secretion of active Wingless was studied in Drosophila. WNT genes encode a large family of secreted proteins; these ligands share a signature WNT motif (C-K-C-H-G-(LIVMT)-S-G-X-C), 22 conserved cysteines, many highly charged amino acid residues, and several potential glycosylation sites. Based on their amino acid sequences, Wnt proteins should be soluble, secreted glycoproteins; yet, Wnts do not exhibit the properties expected of soluble hydrophilic proteins. Drosophila Wingless expression in transgenic S2 cells was studied, and it was found that only about 20% of the secreted protein is present in soluble conditioned medium; the majority of the extracellular Wingless is associated with the cell surface and extracellular matrix (Reichsman, 1996). Recent work Willert (2003) has shown that murine Wnt-3a is palmitoylated at a conserved cysteine residue (Cys-77). Their discovery suggests for the first time that lipid modifications may account for the unusual behavior of Wnt ligands. How palmitoylation affects Wnt-3a signaling and whether or not other Wnts are modified in a similar manner are questions now under investigation. This study provides evidence that Drosophila Wingless is also lipid-modified. In addition, the functional significance of the lipid modification and the role of porcupine in Wingless maturation and secretion was studied (Zhai, 2004).

Many types of proteins (including cytosolic, transmembrane, and secreted proteins) are known to undergo S-palmitoylation, the reversible addition of palmitate to a cysteine via a thioester bond (Linder, 2003). This posttranslational acylation is readily reversible and can regulate both protein localization and function. The addition of the palmitoyl moiety increases protein hydrophobicity and promotes membrane association (Linder, 2003). Palmitoylation also affects intracellular trafficking in that palmitoylated proteins are frequently targeted to specific intracellular organelles as well as to detergent-resistant microdomains (DRMs)1 located at the plasma membrane (Patterson, 2002; Ikonen, 1998). These DRMs, commonly referred to as lipid rafts, are rich in cholesterol and glycosphingolipids and so exist in a separate liquid-ordered phase within the plasma membrane (Simons, 1997). Raft DRMs often form signal transduction centers. The cellular machinery needed for signal transduction becomes organized as some proteins are targeted to rafts, whereas others are excluded. For example, cell surface receptors are sometimes localized to same membrane microdomains where their downstream intracellular partners are also concentrated (Zhai, 2004).

Secreted proteins can also be palmitoylated. The mature form of Hedgehog, Hh-Np, contains two unusual lipid modifications (a C-terminal linked cholesterol moiety and an N-terminal palmitoyl adduct). These lipid modifications increase Hh-Np hydrophobicity, influence apical sorting of the ligand, and likely play a role in the partitioning of Hh-Np into raft DRMs. Skinny Hedgehog, a putative O-acyltransferase, is required for the palmitoylation of Drosophila Hh-Np (Jeong, 2002). Clonal analysis studies suggest that palmitoylation is essential for Hh signaling in the wing disc. In the absence of Skinny Hedgehog activity, Hh-Np is not palmitoylated, and Hedgehog signaling activity is greatly reduced (Zhai, 2004).

Skinny Hedgehog belongs to a diverse family of membrane-bound O-acyltransferases (Hofmann, 2000). Many of these enzymes catalyze the transfer of fatty acids to hydroxyl groups on hydrophobic targets. The target substrates vary widely and include lipids (cholesterol acyltransferase), alginate (AlgI), and waxes (wax synthase). Skinny Hedgehog is somewhat unusual in that it is required for the synthesis of a thioester-linked fatty acid. The chemistry of S-palmitoylation and the palmitoyl reside in the cytosol; others, like Skinny Hedgehog, are thought to function within the ER. Curiously, the vertebrate Sonic Hedgehog protein, Shh-Np, contains an amide-linked palmitoyl group at the N-terminal Cys. It has been suggested that Shh-Np acylation occurs via a two-step mechanism, which is the initial synthesis of a thioester intermediate linking the palmitoyl group to the side chain of the terminal cysteine followed by an intramolecular rearrangement that transfers the palmitoyl group to the N-terminal nitrogen (Pepinsky, 1998). The presence of the amide bond may be functionally significant because an amide bond is far more stable than a thioester linkage (Zhai, 2004).

It is intriguing that porcupine also encodes a putative multipass transmembrane protein belonging to the membrane-bound O-acyltransferase superfamily (Hofmann, 2000; Kadowaki, 1996). porcupine is required for Wingless activity, and Porcupine homologs have been identified in Xenopus, mouse, human, and Caenorhabditis elegans (Tanaka, 2000; Caricasole, 2002; Thorpe, 1997). Genetic and immunocytochemical studies suggest that Porcupine is required for the secretion of active Wingless ligand. porcupine functions upstream and is required by Wingless expressing cells. Wingless is synthesized in porcupine min mutants; however, it accumulates within the expressing cells. All extracellular Wnt-1 staining is absent, suggesting that Wnt-1 secretion is blocked in the porcupine-minus mutant animals (Hofmann, 2000; van den Heuvel, 1993). The Wingless secretion defect can be bypassed by driving overexpression of Wingless. In this case, ectopic Wingless secretion leads to ectopic signaling and death. Thus porcupine has an essential role in directing proper Wingless secretion. Studies with transgenic S2 cells (Tanaka, 2000; Tanaka, 2002) have shown that Porcupine and the N-terminal portion of Wingless can be co-immunoprecipitated in the same complex, suggesting that Porcupine may interact directly with Wingless. Several questions should now be addressed. Does Porcupine mediate a posttranslational modification of Wingless? If so, what is the modification, and why is this modification necessary for normal Wingless secretion (Zhai, 2004)?

This study reports a series of experiments testing the hypothesis that Porcupine is required for lipidation of Wingless. Evidence is presented that Wingless is lipid-modified and that this modification converts Wingless into a membrane-anchored protein that is partitioned into specialized lipid raft microdomains before secretion. Moreover, Porcupine activity is required for both lipidation and the subsequent targeting of Wingless to specialized raft DRMs at the cell surface (Zhai, 2004).

Willert (2002) first demonstrated that murine Wnt-3a is palmitoylated at a conserved cysteine residue and proposed that palmitoyl modifications may be a common feature among Wnt ligands. These studies provide evidence that Drosophila Wingless is also lipidated. Newly synthesized Wingless undergoes a posttranslational modification in which a thioester-linked lipid group is attached to the peptide backbone. Attachment of the lipid affects subsequent Wingless trafficking and secretion in that the lipidated ligand partitions as a membrane-anchored protein and is targeted to lipid raft DRMs before secretion. This lipid modification requires Porcupine, a putative ER membrane-bound O-acyltransferase and can be inhibited by 2-bromopalmitate, a competitive inhibitor of palmitoyl acyltransferases. Lipidation is likely to play a significant role in the maturation and secretion of many if not all Wnt ligands. Recent studies with mammalian cells have shown that Porcupine is required for secretion of murine Wnt-1, -3A, -4, -6 and -7B (Tanaka, 2000). Interestingly, whereas vertebrates have a single porc gene, four different porc transcripts are generated via alternative mRNA splicing (Tanaka, 2000; Caricasole, 2002). Sequence analyses suggest that each transcript encodes a putative acyltransferase; however, it is possible that the isoforms exhibit different lipid and protein substrate specificities (Zhai, 2004).

How important is Wnt lipidation? What is its function? This study has focused on the upstream events that control production and proper secretion of active Wingless ligand. Animals lacking Porcupine activity synthesize Wingless, but the unmodified protein does not associate with membranes and is not secreted. Both defects can be phenocopied in S2 cells by the addition of 2-bromopalmitate. These findings are consistent with a growing body of literature demonstrating that lipidation is often used to regulate protein trafficking. Cooperative lipid-lipid interactions between the lipid moiety and membrane lipids can help target the modified protein to specific vesicles and lipid microdomains. Palmitate groups, which partition with cholesterol and sphingolipids, are important determinants for apical sorting and targeting to raft DRMs (Ikonen, 1998). The membrane-associated Wingless is partitioned into lipid raft DRMs. Lipidation appears to be requisite for raft targeting; the association of Wingless with raft DRM particles is dependent on porcupine activity and can be blocked by treating cells with 2-bromopalmitate. These experiments do not rule out the possibility that Wingless undergoes multiple lipid modifications. Additional palmitate groups or other lipid moieties may be added in subsequent steps. Many proteins require at least two saturated acyl chains for raft targeting (Zhai, 2004).

Based on these results, a pathway is proposed for Wingless maturation and trafficking to the cell surface as follows. Wingless undergoes at least two types of modifications in the ER, which are a Porcupine-dependent fatty acylation and the addition of two Asn-linked high mannose sugars. After glycosylation and lipidation, the protein is transported from the ER to the Golgi where it is partitioned into polarized vesicles and then transported to cholesterol/sphingolipid-rich raft DRMs at the cell surface. In porc mutant animals, Wingless is synthesized and glycosylated, but in the absence of functional Porcupine the ligand is not lipidated. Lacking the appropriate fatty acid sorting signal Wingless does not partition with cholesterol/sphingolipid-rich microdomains and is not transported to the cell surface. Immunocytochemical staining of Wingless expression in porcupine mutant animals suggests that the immature ligand accumulates in the ER and/or early Golgi vesicles (Packard, 2002). Similar results have been reported for apolipoprotein B. Palmitoylation of apolipoprotein B concentrates the protein in specialized compartments within the ER, stimulates ER to Golgi transport, and promotes secretion (Zhai, 2004).

In summary, these data argue that Wingless is not secreted through the classical constitutive pathway. Instead, Wingless undergoes a Porcupine-dependent lipidation; the lipid group functions as a sorting signal, targeting the ligand to polarized vesicles that transport Wingless to unique sites at the cell surface. This pathway is not unlike apical secretion in mammalian cells, where secretory proteins partition with cholesterol-sphingolipid rich microdomains within the trans Golgi network and are then transported directly to the plasma membrane (Zhai, 2004).

After reaching the cell surface, how is Wingless secreted? Numerous immunocytochemical studies indicate that extracellular Wingless is concentrated in large punctate particles (van den Heuvel, 1989; Gonzalez, 1991). Given that extracellular Wnts are lipid modified, secretion is likely to require additional processing steps in which the ligand is either packaged into extracellular secretory vesicles or assembled into lipoprotein aggregates that are then released from the cell surface. The first model is compatible with work by Greco (2001) suggesting that extracellular Wingless is transported via argosomes, extracellular vesicles that travel from cell to cell. Argosome budding and release may take place near lipid raft DRMs. If so, then Porcupine-dependent acylation and subsequent raft targeting may ensure that Wingless is concentrated at the site of argosome budding, perhaps promoting the subsequent packaging of Wingless into argosome vesicles (Zhai, 2004).

These data suggest that Wnt and Hh ligands may utilize similar mechanisms for transport, secretion, and extracellular movement. Hh contains both palmitoyl and cholesterol groups; these lipid modifications target Hh to lipid raft DRMs at the surface. Extracellular Hh-Np is also found in large particles. The assembly of these Hh-Np aggregates depends on cholesterol and dispatched, and movement of the aggregates from the signaling to the receiving cells is dependent on heparan sulfate proteoglycans. In summary, lipid modifications provide important signals for targeting morphogens to specific membrane compartments, and they are likely to participate in the packaging, secretion, and perhaps delivery of the morphogen to the responding cells (Zhai, 2004).

Porcupine-mediated lipid-modification regulates the activity and distribution of Wnt proteins in the chick neural tube

A long-term goal of developmental biology is to understand how morphogens establish gradients that promote proper tissue patterning. A number of reports describe the formation of the Wingless gradient in Drosophila and have shown that Porcupine, a predicted membrane-bound O-acyl transferase, is required for the correct distribution of Wg protein. The discovery that Wnts are palmitoylated on a conserved cysteine residue suggests that porcupine activity and Wnt palmitoylation are important for the generation of Wnt gradients. To establish the role of porcupine in Wnt gradient formation in vertebrates, the role of porcupine/Wnt palmitoylation was tested in human embryonic kidney 293T cells and in the chick neural tube. The results lead to the following conclusions: (1) vertebrate Wnt1 and Wnt3a possess at least one additional site for porcupine-mediated lipid-modification; (2) porcupine-mediated lipid-modification of Wnt proteins promotes their activity in 293T cells and in the chick neural tube; and (3) porcupine-mediated lipid-modification reduces the range of activity of Wnt1 and Wnt3a in the chick neural tube. These findings highlight the importance of porcupine-mediated lipid modifications in the formation of vertebrate Wnt activity gradients (Galli, 2007).

What is the role of palmitoylation of secreted signaling proteins such as Wnts, hedgehogs and Spitz in regulating the activity and distribution of these proteins? Although Hedgehog and Spitz are palmitoylated by the same member of the MBOAT superfamily (Amanai, 2001; Chamoun, 2001; Lee, 2001; Micchelli, 2002; Miura, 2006), this modification has differential effects on the two ligands. Whereas palmitoylation of sonic hedgehog (Shh) is required for long-range signaling (Chen, 2004), palmitoylation of Spitz restricts its range of activity (Miura, 2006). This study shows that porcupine restricts the range of Wnt activity (Galli, 2007).

To reach this conclusion, the role was studied of porcupine in regulating the lipid-modification, activity and distribution of Wnt proteins in vertebrates. First, antibodies against Wnt1 were used to assess distribution of ectopic and endogenous Wnt1 in the developing chick neural tube. When either cWnt1 or cWnt3a protein was overexpressed in the neural tube, the vast majority of the staining was very close to the site of synthesis. Although immunopositive punctae resembling Drosophila vesicular/argosome/lipoprotein-type structures were observed (Gonzalez, 1991; Greco, 2001; Panakova, 2005; Strigini, 2000; van den Heuvel, 1993), the possibility that these represent aggregates of improperly folded Wnt protein cannot be ruled out. Immunostaining for endogenous Wnt1 showed that the protein localized very close to the site of synthesis. A Wnt protein gradient extending as far ventrally as the expression of BAT-gal could not be reliably detected in control embryos. It is suspected that this difference is due to the ability to detect cell-associated, but not secreted, Wnt protein. For instance, tissue fixation precludes the detection of secreted Wg in Drosophila (Strigini, 2000). Similar results were observed with fixed or fresh frozen sections, even though Wnt1 antibody recognizes native Wnt1 protein. The exceptional intractability of Wnt immunostaining is further highlighted by the fact that the immunostaining protocol yields robust staining of the Shh gradient emanating from the floor plate/notochord (Galli, 2007).

Data from a phase-separation assay suggest that ectopic expression of porcupine promotes the lipid-modification of Wnt proteins and that Wnt1/Wnt3a possess at least one additional site for modification by porcupine. These data are consistent with recent reports demonstrating that Wnts are lipid-modified on at least two residues, including C93/C77/C104 of Wnt1/Wnt3a/Wnt5a, respectively (Kurayoshi, 2007; Willert, 2003), and S209 of Wnt3a (Takada, 2006). This observation is significant because membrane interactions generally require two separate lipid modifications in order meet the energetic requirements of the association between protein and membrane. Because these two sites exhibit little sequence similarity, it was asked whether porcupine regulates the modification of both of these sites. Since porcupine is able to promote lipid-modification of the C93/C77 mutants, it is likely that porcupine regulates lipid-modification of the S224/S209 site of Wnt1 and Wnt3a. The observation that porcupine can also promote the modification of Wnt1 when S224 is mutated, further suggests that porcupine regulates the modification of C93. It is not known whether the modification of these sites represents a direct or indirect activity of porcupine. Data showing that the histidine in the acyl transferase active site of porcupine is required for Wnt modification is consistent with the hypothesis that porcupine is the enzyme that directly modifies Wnt proteins. However, the only way to unequivocally test this is to determine whether purified porcupine can mediate lipid-modification of purified Wnt proteins (Galli, 2007).

The role of porcupine/lipid-modification in mediating the activity of Wnt1 and Wnt3a was tested. Consistent with previous studies in Drosophila (Kadowaki, 1996; Nusse, 2003; Siegfried, 1994; Zhai, 2004), porcupine loss-of-function studies in 293T cells and in chick neural tube show a decrease in Wnt activity. Likewise, mutation of C93/C77 and S224/S209 show no detectable activity in 293T cells. In the neural tube, Wnt1(C93S) shows decreased activity, whereas Wnt3a(C77S) shows increased activity. Although the reason the singly mutated Wnt3a is hypermorphic is unknown, it is suspected that mutation of the second lipid-modification site in Wnt3a will result in a loss of activity. Since these studies do not differentiate between the autocrine and paracrine activities of Wnt1 and Wnt3a, it cannot be definitively distinguish whether a decrease in the affinity/avidity of Wnt1/Wnt3a or a decrease in secretion is being observed (Galli, 2007).

It was also found that expression of ectopic mPorcD promotes the activity of cWnt1 and cWnt3a in 293T cells and in the chick dorsal neural tube. This might reflect an increase in affinity/avidity or an increase in secretion. The results from the TX-114 assays showing that porcupine increases the amount of cell-associated Wnt protein, suggest that ectopic porcupine does not simply promote secretion. Furthermore, whereas porcupine loss-of-function experiments cause improper secretion and localization of Wnt proteins (Nusse, 2003; Takada, 2006; van den Heuvel, 1993; Zhai, 2004), gain-of-function experiments show little effect on secretion (Kadowaki, 1996). Thus, it is unlikely that the ability of ectopic porcupine to promote Wnt activity can be solely attributed to increased secretion. It is predicted that lipid-modification of Wnts promotes their affinity/avidity for cell surface receptors. This prediction is consistent with recently published data (Kurayoshi, 2007) showing that binding of Wnt5a to frizzled 5 requires palmitoylation of C104 (Galli, 2007).

Because porcupine is an upstream regulator of glycosylation and lipid-modification of vertebrate Wnt proteins (Kurayoshi, 2007; Takada, 2006; Tanaka, 2002), differences in Wnt activity observed upon porcupine overexpression might stem from changes in glycosylation and not lipidation. Although both glycosylation and lipid-modification are required for the secretion of Wnts, the removal of the two known lipid-modification sites causes a more dramatic loss of activity than the mutation of all asparagine-linked glycosylation sites. As these effects of removing the two lipid-modification sites more closely mimic the effects of knocking down porcupine, it seems likely that the primary role of porcupine with respect to Wnt proteins is the regulation of lipid-modification (Galli, 2007).

The most important findings of this work are derived from the use of the chick model system to assess the effect of porcupine on the Wnt1/Wnt3a gradient in the developing neural tube. Loss-of-function studies identify a requirement for porcupine in the secretion of vertebrate Wnt proteins (Takada, 2006); however, the early requirement for porcupine in secretion precluded determination of the role of porcupine/lipid-modification in regulating the distribution of Wnt protein after secretion. Gain-of-function studies show that porcupine steepens the Wnt-mediated proliferation gradient in the neural tube and shortens the Wnt range of activity as measured by BAT-gal activity. Although the ability of porcupine to steepen the proliferation gradient could be attributed to the ability of porcupine to augment Wnt activity, these results strongly suggest that porcupine limits the distribution of Wnt protein. Thus, it is concluded that porcupine plays a role in regulating both the activity and distribution of Wnt proteins. The interpretation is that lipid-modification by porcupine results in Wnts being tethered closer to their site of synthesis, thus raising their local concentration. Interestingly, heparan sulfate proteoglycans (HSPGs) are also able to promote such an increase in local concentration. Data from extractions showing that porcupine overexpression promotes increased cell-associated Wnt protein is consistent with this idea (Galli, 2007).

Other proteins involved in Wnt secretion and/or the establishment of Wnt gradients have been identified. It will be interesting to determine how these proteins coordinate with porcupine to regulate Wnt secretion and gradient formation (Galli, 2007).

Search PubMed for articles about Drosophila Porcupine

Amanai, K. and Jiang, J. (2001). Distinct roles of Central missing and Dispatched in sending the Hedgehog signal. Development 128: 5119-5127. PubMed ID: 11748147

Caricasole, A., Ferraro, T., Rimland, J. M., and Terstappen, G. C. (2002). Molecular cloning and initial characterization of the MG61/PORC gene, the human homologue of the Drosophila segment polarity gene Porcupine. Gene (Amst.) 288: 147-157. PubMed ID: PubMed ID; Online text

Chamoun, Z., Mann, R. K., Nellen, D., von Kessler, D. P., Bellotto, M., Beachy, P. A. and Basler, K. (2001). Skinny hedgehog, an acyltransferase required for palmitoylation and activity of the hedgehog signal. Science 293: 2080-2084. PubMed ID: PubMed ID; Online text

Chen, M. H., Li, Y. J., Kawakami, T., Xu, S. M. and Chuang, P. T. (2004). Palmitoylation is required for the production of a soluble multimeric Hedgehog protein complex and long-range signaling in vertebrates. Genes Dev. 18: 641-659. PubMed ID: PubMed ID; Online text

Galli, L. M., et al, (2007). Porcupine-mediated lipid-modification regulates the activity and distribution of Wnt proteins in the chick neural tube. Development 134(18): 3339-48. PubMed ID: PubMed ID; Online text

Gonzalez, F., Swales, L., Bejsovec, A., Skaer, H., and Martinez Arias, A. (1991) Mech. Dev. 35: 43-54. PubMed ID: 1720017

Greco, V., Hannus, M. and Eaton, S. (2001). Argosomes: a potential vehicle for the spread of morphogens through epithelia. Cell 106: 633-645. PubMed ID: PubMed ID; Online text

Hofmann, K. (2000). A superfamily of membrane-bound O-acyltransferases with implications for wnt signaling Trends Biochem. Sci. 25: 111-112. PubMed ID: 10694878

Ikonen, E., and Simons, K. (1998). Protein and lipid sorting from the trans-Golgi network to the plasma membrane in polarized cells. Semin. Cell Dev. Biol. 9: 503-509. PubMed ID: PubMed ID; Online text

Jeong, J. and McMahon, A. P. (2002) J. Clin. Investig. 110, 591-596. PubMed ID: PubMed ID; Online text

Kadowaki, T., Wilder, E., Klingensmith, J., Zachary, K. and Perrimon, N. (1996). The segment polarity gene porcupine encodes a putative multitransmembrane protein involved in Wingless processing. Genes Dev. 10: 3116-3128. PubMed ID: PubMed ID; Online text

Kurayoshi, M., Yamamoto, H., Izumi, S. and Kikuchi, A. (2007). Post-translational palmitoylation and glycosylation of Wnt-5a are necessary for its signalling. Biochem. J. 402: 515-523. PubMed ID: PubMed ID; Online text

Lee, J. D. and Treisman, J. E. (2001). Sightless has homology to transmembrane acyltransferases and is required to generate active Hedgehog protein. Curr. Biol. 11: 1147-1152. PubMed ID: PubMed ID; Online text

Linder, M. E., and Deschenes, R. J. (2003). New insights into the mechanisms of protein palmitoylation Biochemistry 42: 4311-4320. PubMed ID: 12693927

Micchelli, C. A., The, I., Selva, E., Mogila, V. and Perrimon, N. (2002). Rasp, a putative transmembrane acyltransferase, is required for Hedgehog signaling. Development 129: 843-851. PubMed ID: 11861468

Miura, G. I., Buglino, J., Alvarado, D., Lemmon, M. A., Resh, M. D. and Treisman, J. E. (2006). Palmitoylation of the EGFR ligand Spitz by Rasp increases Spitz activity by restricting its diffusion. Dev. Cell 10: 167-176. PubMed ID: PubMed ID; Online text

Nusse, R. (2003). Wnts and Hedgehogs: lipid-modified proteins and similarities in signaling mechanisms at the cell surface. Development 130: 5297-5305. PubMed ID: PubMed ID; Online text

Packard, M., Koo, E. S., Gorczyca, M., Sharpe, J., Cumberledge, S., and Budnik, V. (2002). The Drosophila Wnt, wingless, provides an essential signal for pre- and postsynaptic differentiation. Cell 111: 319-330. PubMed ID: 12419243

Panakova, D., Sprong, H., Marois, E., Thiele, C. and Eaton, S. (2005). Lipoprotein particles are required for Hedgehog and Wingless signalling. Nature 435: 58-65. PubMed ID: 15875013

Patterson, S. I. (2002). Posttranslational protein S-palmitoylation and the compartmentalization of signaling molecules in neurons. Biol. Res. 35: 139-150. PubMed ID: 12415731

Pepinsky, R. B., Zeng, C., Wen, D., Rayhorn, P., Baker, D. P., Williams, K. P., Bixler, S. A., Ambrose, C. M., Garber, E. A., Miatkowski, K., Taylor, F. R., Wang, E. A. and Galdes, A. (1998). J. Biol. Chem. 273: 14037-14045. PubMed ID: PubMed ID; Online text

Reichsman, F., Smith, L. and Cumberledge, S. (1996). Glycosaminoglycans can modulate extracellular localization of the wingless protein and promote signal transduction J. Cell Biol. 135: 819-827. PubMed ID: PubMed ID; Online text

Siegfried, E., Wilder, E. L. and Perrimon, N. (1994). Components of wingless signalling in Drosophila. Nature 367: 76-80. PubMed ID: 8107779

Simons, K., and Ikonen, E. (1997). Functional rafts in cell membranes. Nature 387: 569-572. PubMed ID: 9177342

Strigini, M. and Cohen, S. M. (2000). Wingless gradient formation in the Drosophila wing. Curr. Biol. 10: 293-300. PubMed ID: 10744972

Takada, R., Satomi, Y., Kurata, T., Ueno, N., Norioka, S., Kondoh, H., Takao, T. and Takada, S. (2006). Monounsaturated fatty acid modification of Wnt protein: its role in Wnt secretion. Dev. Cell 11: 791-801. PubMed ID: 17141155

Tanaka, K., Okabayashi, K., Asashima, M., Perrimon, N. and Kadowaki, T. (2000). The evolutionarily conserved porcupine gene family is involved in the processing of the Wnt family. Eur. J. Biochem. 267: 4300-4311. PubMed ID: 10866835

Tanaka, K., Kitagawa, Y. and Kadowaki, T. (2002). Drosophila segment polarity gene product porcupine stimulates the posttranslational N-glycosylation of wingless in the endoplasmic reticulum. J. Biol. Chem. 277: 12816-12823. PubMed ID: 11821428

Thorpe, C. J., Schlesinger, A., Carter, J. C., and Bowerman. B. (1997). Wnt signaling polarizes an early C. elegans blastomere to distinguish endoderm from mesoderm. Cell 90, 695-705. PubMed ID: 9288749

van den Heuvel, M., Nusse, R., Johnston, P., and Lawrence, P. A. (1989). Distribution of the wingless gene product in Drosophila embryos: a protein involved in cell-cell communication. Cell 59: 739-749. PubMed ID: 2582493

van den Heuvel, M., Harryman-Samos, C., Klingensmith, J., Perrimon, N. and Nusse, R. (1993). Mutations in the segment polarity genes wingless and porcupine impair secretion of the wingless protein. EMBO J. 12: 5293-5302. PubMed ID: 8262072

Willert, K., Brown, J. D., Danenberg, E., Duncan, A. W., Weissman, I. L., Reya, T., Yates, J. R. and Nusse, R. (2003). Wnt proteins are lipid-modified and can act as stem cell growth factors. Nature 423: 448-452. PubMed ID: 12717451

Zhai, L., Chaturvedi, D. and Cumberledge, S. (2004). Drosophila Wnt-1 undergoes a hydrophobic modification and is targeted to lipid rafts; a process that requires porcupine. J. Biol. Chem. 279: 33220-33227. PubMed ID: PubMed ID; Online text

date revised: 22 February 2008

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