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.

Two C. elegans Yolk Proteins are Cleaved from a Single Precursor Polypeptide

W. Sharrock

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.