Heterosynthetic origin of the major yolk protein, vitellin, in a nereid, Perinereis cultrifera (polychaete annelid)

Abstract

1. 1. An examination of proteins synthesized by Perinereis cultrifera oocytes incubated in vitro with [ 3H]leucine clearly shows that these cells are not capable of synthesizing the main yolk protein previously identified in this worm. 2. 2. In addition, the detection of radiolabelled vitellin in oocytes after in vitro incubation of an oocyte-coelomocyte cell mixture in presence of [ 3H]leucine strongly suggests that the coelomocytes, free cells in the coelomic cavity, synthesize and secrete a vitellin precursor, vitellogenin, that is subsequently taken up by the oocytes. 3. 3. Two native proteins differing in mol. wt but reacting with anti-vitellin antibodies have been identified in coelomocyte incubation medium. Also found in the coelomic fluid, they have been designated VG1 ( M r = 530,000) and VG2 ( M r = 320,000). 4. 4. The two vitellogenins consist of a single type of polypeptide of M r = 176,000 and are incorporated in the oocytes where they are apparently observed under a single molecular form corresponding to VG1, the highest mol. wt protein similar in size to the initial form of vitellin (VI, 530,000). 5. 5. From these data, it seems likely that VG2 is a monomeric molecule that is taken up by the oocytes as a dimer of VG1. 6. 6. We conclude that P. cultrifera accumulates vitellin heterosynthetically and that vitellogenin is produced by the coelomocytes. Moreover, a single polypeptide similar in size to the polypeptidic component of secreted vitellogenin has been detected in the coelomocytes. 7. 7. Since this polypeptide has been identified previously as the single intraoocytic precursor of the four lower mol. wt products that make up the mature form of vitellin (V5), it appears that P. cultrifera exhibits for vitellogenin a processing pathway in which cleavage of the precursor occurs only after uptake by the oocyte.