Developmental fate of the yolk protein lipovitellin in embryos and larvae of winter flounder, Pleuronectes americanus.

The developmental fate of the vitellogenin-derived yolk protein, lipovitellin (Lv), was investigated in winter flounder embryos and yolk-sac larvae. Since Lv is present as only one major polypeptide in ovulated winter flounder eggs, unlike the multiple yolk polypeptides found in the mature eggs of most teleosts, this system is presented as a simpler model of yolk protein structure and utilization during teleostean development. Winter flounder Lv is cleaved during embryogenesis from a 94 kD polypeptide at fertilization to 67 kD and 26 kD polypeptides at hatch- ing. The rate of this proteolytic processing is slow during early embryonic development, but enters a more rapid phase between days 8 and 12 post-fertilization in embryos reared at 4-5°C, and approaches 50% completion at day 10. Lv processing is essentially complete 3 days before hatch- ing; nevertheless, major degradation of the Lv peptide by the developing winter flounder does not occur until after hatching. The Stokes radius of Lv changes only moderately following processing, from 4.50 nm in unfertilized eggs to 4.19 nm in late embryos and newly hatched larvae, whereas the processed Lv retains its heat stability relative to other yolk polypeptides. Nearly 50% of its lipid content, however, is released from the Lv particle during embryogenesis, concomitant with cleavage of the Lv 94 kD polypeptide. Lv processing may thus render a portion of the yolk protein- associated lipid more accessible to the developing embryo, whereas other yolk components are retained for later use by the winter flounder larva. Alternately, removal of lipid may lead to pro- teolytic vulnerability of the Lv polypeptide. In either case, only a portion of the lipid moiety of the Lv particle appears to play a significant nutritive role for the embryo, whereas its protein compo- nent is reserved for larval use. J. Exp. Zool. 284:686-695, 1999. © 1999 Wiley-Liss, Inc. The majority of the yolk resources available to most oviparous animals is derived from vitel- logenin (Vg), a large glycophospholipoprotein. In fishes, as in other vertebrates, Vg is hepatically synthesized and transported via the bloodstream to the ovaries, where it is rapidly and specifically taken up from the blood into maturing oocytes (Selman and Wallace, '82, '83; Tyler et al., '88a,b, '90, '91; Kanungo et al., '90; Tyler, '93) by recep- tor-mediated endocytosis (Opresko and Wiley, '87; Stifani et al., '90). Within oocytes, Vg is proteolyti- cally cleaved to form smaller yolk proteins (de Vlaming et al., '80; Wallace and Begovak, '85; Wallace and Selman, '85; Tyler, '93; Johanning and Specker, '95; Matsubara and Sawano, '95), of which the largest and most abundant, lipovitellin (Lv), contains the lipid-binding region of Vg (re- viewed by Banaszak et al., '91). Piscine oocytes show a remarkable degree of variation from spe- cies to species in the size and in the lipid and phosphorus content of yolk proteins (reviewed by Mommsen and Walsh, '88). Such variability is fur- ther compounded in many teleosts by the addi- tional proteolytic processing of yolk proteins late in oocyte maturation (Wallace and Begovac, '85; Wallace and Selman, '85; Greeley et al., '86; Cerda et al., '96; Thorsen and Fyhn, '96). This phenom- enon is more pronounced in marine fishes, and may help drive oocyte hydration in some (Wallace and Selman, '85; Greeley et al., '86; Carnevali et al., '93; Cerda et al., '96; Thorsen and Fyhn, '96). The oocytes of marine, but not freshwater, fishes have been reported to decrease in protein phos- phorus content in the course of maturation, and this process may be functionally tied to oocyte hy- dration (Craik, '82; Craik and Harvey, '84). Hy- drolysis of yolk protein into free amino acids during final oocyte maturation also creates os-