BIOLOGICAL OVERVIEW

tolloid is intimately involved in the process of dorsal-ventral polarity in Drosophila. The major effector protein molecule in D-V patterning is Decapentaplegic (DPP). DPP is regulated by the Dorsal transcription factor and secreted by dorsal cells. DPP is synthesized in an inactive form and therefore requires enzymatic proteolysis for its activation. Proteolysis refers to the cutting up or digestion of proteins. DPP as originally synthesized is inactive. Undergoing proteolysis allows DPP to become active.

Tolloid protein contains a metalloprotease, and is therefore, a candidate for DPP proteolysis. In addition to a protease domain, TLL possesses two epidermal growth factor repeats and five copies of a repeat found in certain vertebrate complement components. Complement consists of enzymes that kill cells when activated by antibody molecules.

The mammalian bone morphogenetic (BMP) family of proteins consists of seven members. DPP protein is homologous to BMP2/4. Tolloid and Tolloid Related-1 (TR1) are two other members of the BMP family, but unlike DPP and BMP2/4 they are not TGF beta homologs. The BMPs were initially identified as a copurifying mixture of proteins capable of inducing ectopic bone formation in rats. As in rats, it is likely that the BMP family members Tolloid and DPP likewise physically interact.

What is the relationship of TLD with DPP? Does TLL serve to activate DPP? Evidence for a physical interaction between TLD and DPP comes from the observation that certain tld alleles fail to complement a partial loss-of-function dpp allele. This lack of intergenic complementation is not observed in crosses using a deficiency for tld locus, suggesting that certain tld alleles produce aberrant products that form nonfunctional complexes with DPP protein (Ferguson, 1994).

Tolloid enhances DPP function, while Short gastrulation, an ortholog of the Xenopus organizer Chordin, inhibits DPP function. Tolloid is secreted and requires a protelytic processing step for activation. The removal of the N-terminal prodomain is not catalyzed by TLD itself, since it is removed from a putative TLD protease null mutant. Most of the TLD in embryos is in the nonprocessed form. Using epistasis tests and a Xenopus secondary axis induction assay, it has been shown that TLD negates the inhibitory effects of SOG/CHD on DPP/BMP-type ligands. Ventral overexpression in Xenopus of either a dominant negative BMP4 receptor, or a cleavage mutant of either BMP4, Noggin, Chordin, or SOG induces secondary axes in 80%-100% of injected embryos. However, when CHD or SOG mRNA are coinjected, together with an equimolar amount of TLD mRNA, secondary axis induction is blocked, suggesting that TLD is capable of inhibiting SOG or CHD function. Activated TLD is unable to inhibit secondary axis induction mediated by Noggin, the dominant negative BMP receptor, or a cleavage mutant of BMP. In transient transfection assays, TLD cleaves SOG; this cleavage is stimulated by DPP. It is proposed that formation of the embryonic DPP activity gradient involves the opposing effects of SOG inhibiting DPP, and TLD processing SOG to release DPP from the inhibitory complex (Marques, 1997). It thus seems likely that Tolloid targets SOG and not DPP as originally thought.

A recent breakthrough in understanding the DPP-TLD interaction shows that BMP-1, the mammalian TLD homolog acts to cleave not BMP2/4 (the mammalian DPP homolog), but peptides of procollagen. This yields the major fibrous components of vertebrate extracellular matrix. The same action may hold for TLD, and thus it would not act on DPP but on collagen, which might in turn be involved in a DPP activation cascade. Further experiments are needed to clarify the role of TLD in DPP activation (Kessler, 1996).

GENE STRUCTURE

tolloid is closely linked (700 bp) to tolloid related 1, also known as tolkin. Both genes are transcribed in the same direction with the tlr sequence upstream of tld (Nguyen, 1994).

cDNA clone length - 3375

Bases in 5' UTR - 100

Bases in 3' UTR - 101

PROTEIN STRUCTURE

Amino Acids - 1057

Structural Domains

Tolloid exhibits a complex structure consisting of an N-terminal domain with sequence similarity to the astacin family of metalloproteases and a C-terminal domain composed of two EGF-like repeats and five copies of the CUB repeat which was first found in human complement proteins C1r and C1s. The overall structure of TLD is similar to the human bone morphogenetic protein, BMP-1. The tld sequence is 41% identical to human BMP-1 (Shimell, 1991).

date revised: 18 February 2024

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