ISOLATION AND CHARACTERIZATION OF A VITELLOGENIN CDNA FROM RAINBOW-TROUT (ONCORHYNCHUS-MYKISS) AND THE COMPLETE SEQUENCE OF A PHOSVITIN CODING SEGMENT

Phosvitins are extensively phosphorylated serine-rich proteins that ar e derived from a large hepatic phosphoglycolipoprotein, vitellogenin, and are deposited, after suitable processing, in the eggs of oviparous vertebrates, Despite their widespread occurrence and apparent importa nce for early embryonic development, very few phosvitins have been seq uenced thus far, including no teleost protein, We used a polymerase ch ain reaction (PCR)-assisted approach to isolate a vitellogenin cDNA cl one, complementary to mRNA synthesized in the liver of estrogen-treate d rainbow trout, This clone contains a sequence that corresponds to th e composition of a phosvitin previously characterized in our laborator y. The amino terminus was identified by amino acid sequencing of the p rotein. The carboxyl terminus was inferred from homology with other ph osvitin-specific sequences from chicken, Xenopus, and lamprey, Trout p hosvitin appears to be a small, 53-residue-long protein displaying the long runs of serines that are characteristic of all phosvitins descri bed so far.