Title Accumulation, degradation and utilization of yolk proteins in teleosts
Authers Takahiro MATSUBARA, Nobuyuki OHKUBO, Sayumi SAWAGUCHI, and Koichiro GEN
Keywords teleost, egg, vitellogenin, yolk protein, oocyte maturation
Citation Bull. Fish. Res. Agen. No.26, 91-97, 2008
Vitellogenin (Vg) is the precursor of egg yolk proteins in teleost species. Three forms of Vg genes and their transcripts appear to exist in barfin flounder, mosquitofish, and red seabream consisting of two forms of complete Vg (VgA and VgB) and a smaller Vg lacking a phosvitin domain (PvlVg, VgC). From cDNA sequencing analysis of the Vg gene, we clarified that species belonging to higher taxa, including Acanthopterygii and Paracanthopterygii, express all three forms of VgA, VgB and VgC. Both VgA and VgB cleaved into three classes of yolk proteins: lipovitellin, phosvitin and β’-component when they are incorporated into oocytes. VgA and VgB play distinct roles in the regulation of egg buoyancy in barfin flounder through selective proteolysis of their derivative yolk proteins in oocytes undergoing final oocyte maturation. Involvement of a cathepsin B-like enzyme in this maturation associated proteolysis was demonstrated by protease specification procedures. Measurement of ooplasm pH at different stages of oocyte maturation revealed drastic acidification of the ooplasm accompanied with yolk protein proteolysis and activation of cathepsin B. With respect to oocyte hydration for acquisition of egg buoyancy, cytoplasmic maturation events appear to be controlled by changes in the pH in maturating oocytes.
URI http://www.fra.affrc.go.jp/bulletin/bull/bull26/matubara.pdf