Developmentally arrested L2 s(well, possibly L3 s)of the ascarid parasite T. canis can survive at least 9 years in the host, happily evading the immune system. This facility is believed to be attributable partly to the surface coat: a layer of a 120-kDa glycoprotein (TES-120 )which overlies the epicuticle, and is continuously shed from the surface of cultured L2 s. Cloning the Surface Coat Mucin, TES 120 We have demonstrated that TES-120 is almost certainly encoded by a 770 bp hyperabundant mRNA, nmuc1 (nemomucin). The mRNA is trans-spliced with the spliced-leader sequence SL1 and putative nmuc1 protein is 17.6 kDa, increasing in size upon glycosylation. There are three parts: a 16 residue N-terminal hydrophobic signal peptide; an 86 residue mucin-like serine/threonine-rich domain, which contains 72.1% Ser and Thr residues, largely contained in a number of heptamer repeats of the consensus sequence STSSSSA; and a C-terminal cysteine-rich domain. A Surface-Associated Motif This latter comprises 2 repeats of a 36 residue sequence containing a consensus XCXD(X)4C(X)6C(X)12C(X)2TC(X)2C, which we call C6 motifs. Comparing the number of residues of the mucin-like domain with size measurements of several mammalian mucins suggests TES-120 to be 17-20 nm long. Since the surface coat is 10-20 nm thick, it looks as if the surface coat comprises a monolayer of TES-120 .Database searching of GenBank and dbEST databases identified 3 C.elegans sequences containing C6 motifs. Two ESTs from mixed stage cDNA libraries, and the ORF
zk643 .6on LGIII. The two ESTs, like nmuc1 contain paired C-terminal C6 motifs, and N-terminal domains of unknown length. One EST maps to LGV.
zk643 .6comprises five C6 motifs. The nmuc1 mRNA is hyperabundant in L2 s(8.2-13.0% total mRNA!), and carries a 5 SL1 sequence. A second highly abundant cuticular antigen, TES-32 ,contains paired C6 motifs - but at the N-terminus. Given that both TES-32 and TES-120 are surface proteins and have C6 motifs perhaps the C.elegans homologues are also surface proteins? The TES-32 mRNA comprises 0.8-1.9% of total mRNA, and is also trans-spliced with SL1 .Neither mRNA is expressed in adult Toxocara. The non-C6 part of TES-32 is homologous to a family of phosphatidylethanolamine-binding proteins found in yeast, filarial nematodes and mammals - but nobody knows what the functions of any of these proteins really is. Immuno-EM using a MAB specific to TES-32 localizes it to the outer epicuticle, and it is also secreted in large quantities. This MAB coprecipitates TES-120 ,suggesting that TES-32 may anchor TES-120 to the epicuticle. However, glutaraldehyde cross-linking experiments have not confirmed the hypothetical association of these two proteins. Surface Coats and the Glycocalyx The nematode surface coat used to be called the glycocalyx, and a lot of vertebrate epithelial cells also have glycocalyces - electron dense surface layers of highly glycosylated proteins. This applies especially to cells lining ducts (blood vessels, Iymphatics, lungs, gut etc.) Many glycocalyx molecules are small mucins similar to TES-120 (e.g. episialin). Now, some such mucins are overexpressed by tumours - overexpression which is thought to confer immune evasory properties to the tumours. This begs the question: could the mechanisms of mucin-mediated immune evasion by some tumours and some nematodes be one and the same?