Worm Breeder's Gazette 8(1): 43
These abstracts should not be cited in bibliographies. Material contained herein should be treated as personal communication and should be cited as such only with the consent of the author.
C. elegans makes four yolk proteins: two closely related polypeptides of approximately 170,000 daltons and two apparently unrelated proteins of 115,000 and 88,000 daltons (Sharrock, 1983, Develop. Biol. 96, 182-188). For some time, the origins of the two smaller yolk proteins have remained obscure. I have consistently found that translation of C. elegans RNA in vitro yields a strong band at approximately 170,000 daltons but nothing corresponding to yp115 or yp88. This paradox has now been resolved by an analysis which exploits the specificity of antisera raised against the purified yolk proteins. Running translation products on a 5% gel, I found three bands in the range of 170,000 to 180,000 daltons. Two of these bands comigrate with the mature yp170 doublet; the third, which migrates more slowly than the doublet (call it 180,000 daltons), has several properties which have led me to conclude that it represents a precursor of both yp115 and yp88. The putative 180K precursor is bound by both anti-yp115 and anti- yp88 IgGs in Protein A-mediated immunoadsorption experiments. Furthermore, the immunoadsorbed material yields the same pattern of fragments when digested with S. aureus V8 protease, regardless of whether anti-yp115 or anti-yp88 IgG is used in the adsorption. Thus, both IgGs are recognizing the same polypeptide. The 180K band may be detected in homogenates of nematodes labelled in vivo, but, as expected for a precursor, it is metabolically unstable. A puzzling aspect of these results is that the molecular weights don't add up: 115,000 + 88,000 = 203,000, which is considerably more than the 180, 000 daltons estimated for the precursor. This discrepancy may be due to post-translational modifications. Modified proteins are known to exhibit anomalous migration in polyacrylamide gels. The true polypeptide molecular weights of yp115 and yp88 may be less than those assigned on the basis of electrophoretic mobility. Interestingly, the V8 protease digestion pattern of the precursor has a few bands in common with those of either yp115 or yp88. This may be due to the translation product's lacking modifications which are present on the mature yolk proteins. Cleavage of the precursor might also alter local peptide conformation, affecting the accessibility of cleavage sites. In contrast, the yp170 translation products yield fragment patterns virtually indistinguishable from those of the mature proteins. It seems then, that C. elegans employs two different pathways for the production of yolk proteins. The cleavage of yp115 and yp88 from a large precursor is reminiscent of the vitellogenin-vitellin pathways of many insects and egg-laying vertebrates. The two yp170s, on the other hand, apparently find their way to the egg with a minimum of processing, in this respect resembling the considerably smaller yolk proteins of Drosophila.