Multiple vitellogenins and their unique roles in marine teleosts

Abstract

Vitellogenin (Vg) is the precursor to egg yolk proteins of teleost fishes, including lipovitellin (Lv), phosvitin (Pv), and β′-component (β′-c). Complete Vg molecules contain the yolk protein domains arranged in linear fashion: NH2-Lv heavy chain, Pv, Lv light chain, β′-c, C-terminal coding region (C-t)-COOH. Western blots employing an antiserum raised against a recombinant C-t polypeptide revealed that the C-t domain of Vg gives rise to a fourth yolk protein in barfin flounder oocytes. Three classes of Vg appear to exist in teleosts, including two types of complete Vg (A-type and B-type) and a smaller, incomplete Vg lacking a Pv domain (Pv-less type). By sequencing cDNA amplified from distinctive regions of each type of Vg transcript from several teleosts, we discovered that members of higher taxa (e.g. Paracanthopterygii and Acanthopterygii) express both Vg A and Vg B, and that Pv-less Vg is widely present among teleosts. Vitellogenins A and B play distinct roles in regulation of egg buoyancy in barfin flounder through selective proteolysis of their product yolk proteins in oocytes undergoing final maturation. Protease specification procedures confirmed involvement of a cathepsin B-like enzyme in this maturation-associated proteolysis. Measurement of changing pH revealed that drastic acidification of the ooplasm accompanies hydrolysis of the yolk proteins. Thus, with respect to yolk protein hydrolysis and oocyte hydration, cytoplasmic maturation appears to be controlled by changing pH in maturing oocytes.