Yolk protein in leech. Identification, purification and characterization of vitellin and vitellogenin

Theromyzon tessulatum vitellin was identified as a lipoglycoprotein of 490 kDa. The insolubility of this molecule in low-ionic-strength media was used to extract it from the ovaries. Antiserum prepared against vitellin was shown to react with a coelomic fluid component of 520 kDa. This vitellin precursor, or vitellogenin, was purified by gel permeation and ion-exchange column chromatography. These two lipoglycoproteins were characterized by amino acid, carbohydrate and lipid analysis and subunit composition. In spite of differences in terms of native molecular mass, solubility and isoelectric point, the lipoglycoproteins isolated from the coelomic fluid and the ovary were similar in their subunit components (a single polypeptide of 165 kDa) and in their amino acid and carbohydrate compositions. However, vitellogenin was found to be more highly lipidated (31.8% by mass) than vitellin (24% by mass) and lipid analysis indicated a higher amount of sterols and phospholipids in vitellogenin. From these data, we conclude that vitellogenin and vitellin are probably dimers of two identical subunit polypeptides plus lipid and that, after vitellogenin is sequestered in the oocyte, part of its lipid component is stripped from the molecule to give vitellin. Furthermore, electrophoretic analysis seems to indicate that vitellogenin synthesis and secretion is initiated following the third and last blood meal of the animal but that vitellogenin significantly accumulates in the coelomic fluid before being incorporated in the oocytes suggesting a complex mode of vitellogenesis regulation.