InteractiveFly: GeneBrief

Golgin, RAB6 interacting: Biological Overview | References |

The duplication and 9-fold symmetry of the Drosophila centriole requires that the cartwheel molecule, Sas6, physically associates with Gorab, a trans-Golgi component. How Gorab achieves these disparate associations is unclear. This study used hydrogen-deuterium exchange mass spectrometry to define Gorab's interacting surfaces that mediate its sub-cellular localization. A core stabilization sequence within Gorab's C-terminal coiled-coil domain was identified that enables homodimerization, binding to Rab6, and thereby trans-Golgi localization. By contrast, part of the Gorab monomer's coiled-coil domain undergoes an anti-parallel interaction with a segment of the parallel coiled-coil dimer of Sas6. This stable hetero-trimeric complex can be visualized by electron microscopy. Mutation of a single leucine residue in Sas6's Gorab-binding domain generates a Sas6 variant with a 16-fold reduced binding affinity for Gorab that can not support centriole duplication. Thus Gorab dimers at the Golgi exist in equilibrium with Sas-6 associated monomers at the centriole to balance Gorab's dual role (Fatalska, 2021).

Centrioles are the ninefold symmetrical microtubule arrays found at the core of centrosomes, the bodies that organize cytoplasmic microtubules in interphase and mitosis. Centrioles also serve as the basal bodies of both non-motile and motile cilia, and flagellae. The core components of centrioles and the molecules that regulate their assembly are highly conserved. The initiation of centriole duplication first requires that the mother and daughter pair of centrioles at each spindle pole disengage at the end of mitosis. Plk4 then phosphorylates Ana2 (Drosophila)/STIL(human) at its N-terminal part, which promotes Ana2 recruitment to the site of procentriole formation, and at its C-terminal part, which enables Ana2 to bind and thereby recruit Sas6. The ensuing assembly of a ninefold symmetrical arrangement of Sas6 dimers provides the structural basis for the ninefold symmetrical cartwheel structure at the procentriole's core. Sas6 interacts with Cep135 and in turn with Sas4 (Drosophila)/CPAP (human), which provides the linkage to centriole microtubules (Fatalska, 2021).

An unexpected requirement has been identified for the protein, Gorab, to establish the ninefold symmetry of centrioles (Kovacs, 2018). Flies lacking Gorab are uncoordinated due to basal body defects in sensory cilia, which lose their ninefold symmetry, and also exhibit maternal effect lethality due to failure of centriole duplication in the syncytial embryo. Gorab is a trans-Golgi-associated protein. Its human counterpart is mutated in the wrinkly skin disease, gerodermia osteodysplastica. By copying a missense mutation in gerodermia patients that disrupts the association of Gorab with the Golgi, this study was able to create mutant Drosophila Gorab, which was also unable to localize to trans-Golgi. However, this mutant form of Gorab was still able to rescue the centriole and cilia defects of gorab null flies. It was also found that expression of C-terminally tagged Gorab disrupts Golgi functions in cytokinesis of male meiosis, a dominant phenotype that can be overcome by a second mutation preventing Golgi targeting. Thus, centriole and Golgi functions of Drosophila Gorab are separable (Fatalska, 2021).

The Golgi apparatus both delivers and receives vesicles to and from multiple cellular destinations and is also responsible for modifying proteins and lipids. Gorab resembles a group of homodimeric rod-like proteins, the golgins, which function in vesicle tethering. The golgins associate through their C-termini with different Golgi domains, and their N-termini both capture vesicles and provide specificity to their tethering. There is known redundancy of golgin function, reflected by the overlapping specificity of the types of vesicles they capture. Gorab is rapidly displaced from the trans-side of the Golgi apparatus by Brefeldin A, suggesting that its peripheral membrane association requires ARF-GTPase activity (Fatalska, 2021).

Previous studies of human Gorab indicated its ability to form a homodimer in complex with Rab6 and identified its putative coiled-coil region as a requirement to localize at the trans-Golgi (Egerer, 2015; Witkos, 2019). Studies on its Drosophila counterpart supported Gorab's ability to interact with itself, potentially through the predicted coiled-coil motif. However, this region was also found to overlap with the region required for Gorab's interaction with Sas6 (Kovacs, 2018). These findings raised the questions of how Gorab's putative coiled-coil region could facilitate interactions with the Golgi, on the one hand, and its Sas6 partner, on the other. To address this, s hydrogen-deuterium exchange (HDX) was employed in conjunction with mass spectrometry (MS). HDX enables the identification of dynamic features of protein by monitoring the exchange of main chain amide protons to deuteria in solution. This study used HDX-MS to monitor the retarded exchange of amide protons localized between interacting regions of Gorab and Sas6 to identify the interacting surfaces within the Gorab-Sas6 complex. Together with other biophysical characterizations, this has revealed that Gorab is able to form a homo-dimer through its coiled-coil region but that it interacts as a monomer with the C-terminal coiled-coil of Sas6. Mutation of a critical amino acid in Sas6's Gorab-binding domain generates a variant of Sas6 with a sixteenfold reduced binding affinity for Gorab that is no longer able to support centriole duplication (Fatalska, 2021).

Together, these findings indicate that Gorab exists at the trans-Golgi network as a homodimer. Dimerization requires its coiled-coil motif (residues 200-315) within which is a core sequence (residues 270-287) that represents the most stable part of this dimerization region. Dimerization enables Gorab to interact with Rab6, and this in turn enables its association with the trans-Golgi. In contrast, Gorab interacts with Sas6 as a monomer. Gorab's binding to Sas6 occurs with a higher affinity than its homodimerization, enabling a Gorab monomer to associate with the Sas6 dimer. Thus, the relatively small number of Sas6 molecules at the centriole would more avidly bind the Gorab monomer, allowing greater excess of Gorab to accumulate as dimers at the trans-Golgi. Sas6 and Gorab interact through short interfaces within their coiled-coil regions. Disruption of this region of Sas6 through mutation of a single conserved leucine residue, L447, results in a failure of Gorab to bind to Sas6 and localize to the centriole. While the possibility cannot be formally excluded that the L447A mutation affects some other aspect of Sas6 function, the finding that expression of this mutant phenocopies a strong gorab hypomorph in its effects upon both co-ordination and centriole duplication suggests that failure to recruit Gorab is responsible for the Sas6-L447A defect. The finding of some residual apparent Gorab-like function in Sas6-L447A-expressing flies may reflect the overexpression of the protein due to the technical requirements of the experiment and the fact that Sas6-L447A still binds Gorab but with a sixteenfold reduced affinity compared to wild-type Sas6. Given that Sas6-L447A greatly diminishes the interaction with Gorab, whereas the mutation, M440A, in the adjoining 'a' position of the 'a-g' coiled-coil heptad repeat does not, leads to the conclusion that Gorab binds to a narrow region near the C-terminus of the coiled coil of Sas6 (Fatalska, 2021).

Gorab shows many of the properties typical of golgins, a family of tentacle-like proteins that protrude from the Golgi membranes to capture a variety of target vesicles. Redundancy between golgins in their ability to bind target vesicles could act as a functional safeguard and might explain why loss-of-function gorab mutants display no obvious Golgi phenotype, contrasting to the Golgi defects shown by the C-terminally tagged Gorab molecule (Kovacs, 2018). Gorab is similar to other golgins, which also associate with the Golgi membranes through their C-terminal parts in interactions that require Rab family member proteins to interact with the C-terminal part of the golgin dimer. The N-terminal parts of the golgins interact with their vesicle targets. Human GORAB's N-terminal part interacts with Scyl1 to promote the formation of COPI vesicles at the trans-Golgi (Witkos, 2019). However, its precise role in the transport of COPI vesicles is not clear, particularly why loss of human GORAB affects Golgi functions in just bone and skin when COPI function is required in multiple tissues. Drosophila Gorab also co-purifies and physically interacts with both Yata, counterpart of Scyl1, and COPI vesicle components, and its importance for transport of COPI vesicles in Drosophila is similarly unclear (Fatalska, 2021).

This study offers a perspective on how Gorab interacts with Sas6 at the centriole and suggests the possibilities for why this interaction is essential to establish the centriole's ninefold symmetry. The heterotrimeric structure formed by a Sas6 dimer and the Gorab monomer will together constitute a single spoke plus central hub unit of the centriole's cartwheel. The C-terminal part of Gorab would be expected to lie in a tight antiparallel association with the C-terminal part of Sas6's coiled-coil region. Gorab's N-terminus might thus be expected to extend towards the centriolar microtubules and their associated proteins. As the microtubules of Drosophila's somatic centrioles exist as doublets of A- and B-tubules, it is tempting to speculate that Gorab interacts with the centriole wall in a region occupied in other cell types by the C-tubule. This could account for the lack of any requirement for Sas6-Gorab interaction in the male germ-line, where centrioles have triplet microtubules and a C-tubule occupies this space. Gorab's partner proteins interacting with its N-terminal region are therefore of great interest at both the Golgi and in the centriole, and it will be key to understand the nature of these interactions in future studies (Fatalska, 2021).

Interaction interface in the C-terminal parts of centriole proteins Sas6 and Ana2

The centriole is a ninefold symmetrical structure found at the core of centrosomes and, as a basal body, at the base of cilia, whose conserved duplication is regulated by Plk4 kinase. Plk4 phosphorylates a single serine residue at the N-terminus of Ana2 to promote Ana2's loading to the site of procentriole formation. Four conserved serines in Ana2's STAN motif are then phosphorylated by Plk4, enabling Sas6 recruitment. Crystallographic data indicate that the coiled-coil domain of Ana2 forms a tetramer but the structure of full-length Ana2 has not been solved. This study employed hydrogen-deuterium exchange coupled with mass spectrometry (HDX-MS) to uncover the conformational dynamics of Ana2, revealing the high flexibility of this protein with one rigid region. To determine the elusive nature of the interaction surfaces between Ana2 and Sas6, we have confirmed complex formation between the phosphomimetic form of Ana2 (Ana2-4D) and Sas6 in vitro and in vivo. Analysis of this complex by HDX-MS identifies short critical regions required for this interaction, which lie in the C-terminal parts of both proteins. Mutational studies confirmed the relevance of these regions for the Ana2-Sas6 interaction. The Sas6 site required for Ana2 binding is distinct from the site required for Sas6 to bind Gorab and Sas6 is able to bind both these protein partners simultaneously (Fatalska, 2020).

Gorab is a Golgi protein required for structure and duplication of Drosophila centrioles

This study demonstrated that a Drosophila Golgi protein, Gorab, is present not only in the trans-Golgi but also in the centriole cartwheel where, complexed to Sas6, it is required for centriole duplication. In addition to centriole defects, flies lacking Gorab are uncoordinated due to defects in sensory cilia, which lose their nine-fold symmetry. The separation of centriole and Golgi functions of Drosophila Gorab were demonstrated in two ways: first, Gorab variants were created that are unable to localize to trans-Golgi but can still rescue the centriole and cilia defects of gorab null flies; second, it was shown that expression of C-terminally tagged Gorab disrupts Golgi functions in cytokinesis of male meiosis, a dominant phenotype overcome by mutations preventing Golgi targeting. These findings suggest that during animal evolution, a Golgi protein has arisen with a second, apparently independent, role in centriole duplication (Kovacs, 2018).

This study has identified a tissue specific role for Golgi-associated Gorab in centriole duplication in Drosophila. Gorab physically interacts with the centriole cartwheel component, Sas6, with which it co-localizes from the onset of procentriole formation. Centrosomes fail to duplicate in gorab-mutant-derived embryos and in diploid tissues of gorab-null Drosophila, which lose coordination through defects in their mechanosensory cilia. Such cilia have a single, mother centriole-derived basal body with six to ten sets of microtubules, and this abnormal symmetry extends into the ciliary axoneme (Kovacs, 2018).

Loss of nine-fold symmetry in gorab-mutant centrioles is reminiscent of Sas6 mutants. It suggests the Gorab–Sas6 partnership is required for both centriole duplication and symmetry. The formation of centrioles with correct symmetry can still be directed around Sas6 variants that are unable to establish nine-fold symmetry. This suggests other components of the centriole, in addition to Sas6, also contribute to its symmetry. Gorab could be one such contributing molecule, at least in part. However, this cannot be universally true, because centrioles and axonemes in the gonads of fully fertile gorab-null males have correct nine-fold symmetry. This could be either because maternal Gorab protein perdures sufficiently in male germ cells to permit centriole duplication or because Gorab is substituted by another molecule in spermatogenesis. These possibilities, either of which could reflect the distinctive morphology of Drosophila's spermatocyte centrioles, require further study (Kovacs, 2018).

The trans-Golgi localization of human GORAB is mirrored in multiple Drosophila tissues, including salivary glands, imaginal discs, the central nervous system, and in the male and female germ lines, but not in syncytial embryos, where Golgi has yet to form. Accordingly, Gorab's association with COPI coatomer components in cultured cells suggests involvement in retrograde vesicle transport from Golgi to ER consistent with its resemblance to a golgin. The rod-like golgins, which bind Rab, Arf, or ADP-ribosylation family GTPases, are tethered to Golgi membranes by their C termini and protrude outwards to capture vesicles at their N termini. Overlapping specificity in vesicle targeting provides redundancy of function. Thus, both golgins GMAP-210 and GM130 can capture ER-derived carriers; both GMAP-210 and Golgin-84 can capture cis-Golgi derived vesicles; and so on. Such redundancy might account for the lack of any Golgi phenotype in gorab-null mutants. However, Gorab's functional relevance at Drosophila Golgi is indicated by the cytokinesis defects in male meiosis caused by expression of C-terminally tagged Gorab, which are strikingly similar to those following disruption of COPI-mediated vesicle trafficking. This accords with Gorab's association with COPI proteins and reinforces suggestions that the integrity of ER and other membranous structures is interdependent with astral and spindle microtubule function in male meiosis (Kovacs, 2018).

By generating the counterpart of a gerodermia osteodysplastica missense mutant that prevents human GORAB from localizing to Golgi, Drosophila Gorab's Golgi and centriole functions can be separated. This p.Val266Pro mutation prevents Gorab from associating with Golgi but fully rescues centriole duplication defects of gorab-null mutants and restores their ciliary function. A proline residue at this site could strongly influence structure of the Golgi-interacting region because of its side chain's rigidity and ability to undergo cis–trans isomerization. The mutation did not, however, interfere with Gorab's ability to bind Sas6. Moreover, introducing p.Val266Pro into C-terminally tagged Gorab prevented its localization to Golgi and so relieved the cytokinesis defect. Thus the male sterility resulting from a C-terminal GFP tag is mediated through Gorab's Golgi association. Gorab's precise Golgi functions in Drosophila, most likely redundant with other golgins, must await further genetic and molecular studies (Kovacs, 2018).

Gorab is not required for centriole duplication or Golgi function in unicellular organisms such as ciliated eukaryotes. Its evolutionary appearance in animals may reflect increased proximity and functional interactions between the Golgi, centrosomes, and cilia. Such co-evolution could have facilitated the emergence of proteins with dual functions, allowing a component of one organelle to take on an additional function in its neighbor. However, Gorab is not present in all animal species; it is absent, for example, from C. elegans. This could possibly reflect the assembly of C. elegans SAS6 into a spiral rather than the ring-shaped oligomers characteristic of the centriole cartwheels in most species, which may obviate the need for interactions with a Gorab-like protein. Moreover, even within a single species, Gorab may be required for centriole duplication in some tissues and not others, as was found in Drosophila. Such tissue specificity might account for findings with a GORAB-mutant mouse, which has few primary cilia in dermal condensate cells responsible for Hedgehog signaling in hair follicles but does have primary cilia on keratinocytes. This could reflect tissue-specific failure of centriole duplication in the GORAB-null mouse, even though there is currently no evidence to support this notion (Kovacs, 2018).

Although the above defects in cilia development in the GORAB-null mouse require molecular analysis, they suggest a possibility of conserved roles for GORAB. Both fly and human proteins are not only found at the trans-Golgi but also at the centriole. GFP-tagged GORAB was expressed in U2OS cells, and it was found at both centrosomes and Golgi. The Golgi localization was abolished by the p.Ala220Pro mutation but centrosome association remained. This study also found that anti-GORAB antibodies could detect human GORAB at the centriole, albeit not always together with Sas6 as in Drosophila. This might reflect different requirements for GORAB and Sas6 at the centriole in the two organisms; Sas6 remains centriole-associated throughout the Drosophila duplication cycle, whereas it is first recruited and then is later absent from the lumen of the mother centriole for a substantial part of the human duplication cycle. It will be of future interest to track the precise behaviors of SAS6 and GORAB throughout the centriole duplication cycle in human cells (Kovacs, 2018).

Currently, however, it remains uncertain whether GORAB functions in centriole duplication in human cells as in insects. Because mammalian cells lacking centrosomes are prevented from cell cycle progression by a p53-dependent pathway, attempts were made to assess the consequences of GORAB depletion on centrosome number in a human osteosarcoma (U2OS) line expressing dominant-negative p53 (U2OS p53DD). It is found that some more-stable centriole proteins require more rounds of knockdown before a duplication phenotype can be observed and, unfortunately, GORAB RNAi led to cell death before depletion was complete. This was possibly due to compromised Golgi function, making it difficult to assess the effect upon centriole duplication. However, GORAB RNAi enhanced the centrosome loss seen after depletion of SASS6 alone, suggesting the possibility of a cooperative role between the two proteins. It was also found that depletion of human GORAB abolished the centrosome overduplication that occurs in U2OS cells held in S-phase following aphidicolin and hydroxyurea treatment. However, because Golgi function is also compromised by these treatments, it is not certain that human GORAB is required for centrosome duplication as in flies (Kovacs, 2018).

It is noted that the p.Ala220Pro mutation results in a disease phenotype comparable to null mutations8. As GORAB p.Ala220Pro can still associate with the centrosome, this suggests that the gerodermia osteodysplastica phenotype is likely to result predominantly from defective Golgi functioning. However, it would still be worthwhile to re-examine cells from different tissues of patients with GORAB null mutations for potential additional defects in centriole duplication and/or formation of primary cilia. It will also be important to examine Gorab−/− mice further to determine whether the reported loss of cilia could arise through failure of centriole duplication rather than as a secondary consequence of Golgi malfunction (Kovacs, 2018).

To conclude, these findings bring insight into the dual life of a protein with Golgi and centriole functions but also raise new future questions. An understanding of the precise role of Gorab at the Golgi awaits a greater knowledge of Gorab's Golgi partners and its redundancy with other golgins. Moreover, full understanding of Gorab's centriole duplication function in Drosophila awaits future studies of its precise structural interactions with Sas6 and other centriole proteins (Kovacs, 2018).

Search PubMed for articles about Drosophila Gorab

Egerer, J., Emmerich, D., Fischer-Zirnsak, B., Chan, W. L., Meierhofer, D., Tuysuz, B., Marschner, K., Sauer, S., Barr, F. A., Mundlos, S. and Kornak, U. (2015). GORAB missense mutations disrupt RAB6 and ARF5 binding and Golgi targeting. J Invest Dermatol 135(10): 2368-2376. PubMed ID: 26000619

Fatalska, A., Stepinac, E., Richter, M., Kovacs, L., Pietras, Z., Puchinger, M., Dong, G., Dadlez, M. and Glover, D. M. (2021). The dimeric Golgi protein Gorab binds to Sas6 as a monomer to mediate centriole duplication. Elife 10. PubMed ID: 33704067

Fatalska, A., Dzhindzhev, N. S., Dadlez, M. and Glover, D. M. (2020). Interaction interface in the C-terminal parts of centriole proteins Sas6 and Ana2. Open Biol 10(11): 200221. PubMed ID: 33171067

Kovacs, L., Chao-Chu, J., Schneider, S., Gottardo, M., Tzolovsky, G., Dzhindzhev, N. S., Riparbelli, M. G., Callaini, G. and Glover, D. M. (2018). Gorab is a Golgi protein required for structure and duplication of Drosophila centrioles. Nat Genet. PubMed ID: 29892014

Liu, Y., Chen, X., Choi, Y. J., Yang, N., Song, Z., Snedecor, E. R., Liang, W., Leung, E. L., Zhang, L., Qin, C. and Chen, J. (2020). GORAB promotes embryonic lung maturation through antagonizing AKT phosphorylation, versican expression, and mesenchymal cell migration. FASEB J 34(4): 4918-4933. PubMed ID: 32067289

Witkos, T. M., Chan, W. L., Joensuu, M., Rhiel, M., Pallister, E., Thomas-Oates, J., Mould, A. P., Mironov, A. A., Biot, C., Guerardel, Y., Morelle, W., Ungar, D., Wieland, F. T., Jokitalo, E., Tassabehji, M., Kornak, U. and Lowe, M. (2019). GORAB scaffolds COPI at the trans-Golgi for efficient enzyme recycling and correct protein glycosylation. Nat Commun 10(1): 127. PubMed ID: 30631079

date revised: 11 November 2021

Home page: The Interactive Fly © 2011 Thomas Brody, Ph.D.