Derivation of Major Yolk Proteins from Parental Vitellogenins and Alternative Processing During Oocyte Maturation inFundulus heteroclitus

Various Coomassie blue-staining yolk proteins (YPs) present in oocytes and eggs of Fundulus heteroclitus, a teleost that pro-duces low hydrated, demersal eggs (benthophil species), were subjected to N-terminal microsequencing. Four YPs were N-ter-minally blocked, while five yielded sequence information. Of the latter, four corresponded to internal sequences of vitello-genin 1 (Vg1), whereas a fifth band corresponded to the N-terminal sequence of Vg2. Phosphorylated YPs (phosvitins and phosvettes) derived from the polyserine domain of Vg were not successfully sequenced. The major N-terminally blocked 122-and 103-kDa YPs both represented the lipovitellin heavy chain of Vg1 (LvH1), and thus most of the oocyte YPs were derived from Vg1. During oocyte maturation in vivo and in vitro, the LvH1 122 is degraded, concomitant with an increased enzy-matic activity of cathepsin B, while the 45-kDa YP is converted to a 42-kDa YP. The LvH1 122 was found to contain a consensus site for proteolytic degradation (PEST) near its C-terminus, which is missing from its stable, but truncated twin sequence, LvH1 103. We suggest that this site becomes exposed to ca-thepsin B during the hydration process that accompanies oocyte maturation and renders the LvH1 122 susceptible to proteolysis. PEST sites are found in Vg sequences from other benthophil fish, whereas, interestingly, they are missing in marine teleosts that spawn highly hydrated, pelagic eggs (pelagophil species), dis-playing a different pattern of Vg incorporation into YPs and LvH1 and LvH2 processing to that found inF. heteroclitus. Thus, different models of Vg/YP precursor/product relationship and further processing during oocyte maturation and hydration are proposed for pelagophil and benthophil teleosts.