Formation of mature egg envelope subunit proteins from their precursors (choriogenins) in the fish, Oryzias latipes: Loss of partial C-terminal sequences of the choriogenins

The inner layer of egg envelope of the medaka, Oryzias latipes, comprises t wo major groups of glycoprotein subunits, ZI-1,2 and ZI-3, Their precursor proteins, choriogenin H (Chg H) and choriogenin L (Chg L), respectively, ar e synthesized in spawning female liver. In the present study, the primary s tructures of the precursors and the corresponding mature subunits were comp ared by peptide mapping and amino acid sequencing to find what difference i n their molecular structures is relevant to the assembly of the soluble pre cursors into the insoluble inner layer. The primary structures of the solub ilized subunits were mostly identical to those of the respective precursors , but they lacked C-terminal partial sequences that their precursors posses sed, namely, ZI-1,2 subunit was shorter than Chg H by 34 amino acid residue s and ZI-3 was shorter than Chg L by 27 residues. In addition, a consensus amino acid sequence, Arg-Lys-X-Arg, was found at the putative cleavage site s in the C-terminal region of the precursors. It is conjectured that the tr uncation of the precursor proteins is prerequisite for formation of mature chorion subunit proteins and their assembly into chorion.