Based on the amino acid sequence of a cDNA encoding the major larval surface protein of the parasitic nematode Toxocara canis 1 , plus studies of T. canis surface proteins, it is likely that nematode surface proteins are structurally similar to the mucins that line and protect epithelial cell layers in vertebrate animals. Mucins have two characteristic features: 1. domains rich in serine and threonine, which are extensively O-glycosylated; 2. domains rich in cysteine, which form inter- or intra-molecular disulfide bonds. These features confer special physical and biochemical properties on mucins, such as high solution viscosity and resistance to proteolysis. The epitope recognized by monoclonal antibody M38 is detected on the surface of live C. elegans L1s, and the timing of its expression is altered in
srf-6 mutants, which display it in stages L1 through L4. We demonstrated that the M38 epitope is carried by a 30 kD antigen detected in extracts of C. elegans L1s 2 . The antigenicity of this protein is destroyed by pretreatment with O-glycanase, suggesting that it is an O-linked glycoprotein, like the T. canis TES-120 surface antigen. A nematode-specific gene family in C. elegans has been described, members of which encode adjacent serine-threonine-rich and cysteine-rich (SXC or six-cysteine) domains 3 . Because of the likelihood that these encode surface proteins, we have begun to characterize their expression by RT-PCR. For initial studies, we have chosen the predicted genomic coding sequence F41G3.10, which contains 10 exons, each of which encodes a ser-thr-rich sequence followed by an SXC domain. In initial experiments using gene-specific primers, we have amplified a small cDNA fragment spanning 3' terminal exons 9 and 10 in total RNA from mixed stages of C. elegans. We plan to study the stage-specificity of expression of this RNA, as well as map a full-length cDNA representing this transcript. 1 Gems, D. and Maizels, R.M. (1996) Proc. Natl. Acad. Sci. USA (1996) 93: 1665-1670. 2 Hemmer, R.M., Donkin, S.G., Chin, K.J., Grenache, D.G., Bhatt, H., and Politz, S.M. (1991) J. Cell Biol. 115: 1237-1247. 3 Blaxter, M. (1999) Science 282: 2041-2046.