Studies on the assembly and secretion of a multicomponent protein. The biosynthesis of vitellogenin, an oestrogen-induced glycolipophosphoprotein

1. Incorporation of (32)P]P(i) and (3)H]leucine into vitellogenin secreted in vitro by liver slices from oestrogen-treated Xenopus laevis is accompanied by a 2h lag; no lag is apparent for the incorporation into total tissue protein. 2. The addition of cycloheximide was found immediately to inhibit further incorporation of radioactive leucine into total tissue protein. The incorporation into secreted vitellogenin, however, continued for 2h after the addition of cycloheximide. 3. Pulse-labelling of liver slices with (3)H]leucine for 30min, followed by a chase with a large excess of unlabelled leucine, resulted in the appearance of radioactivity in secreted vitellogenin from 90min after the end of the pulse period. 4. Evidence is presented which suggests that of the radioactivity from (3)H]leucine incorporated into proteins by the liver of oestrogen-treated Xenopus some 70% is present in the single protein vitellogenin. 5. The incorporation of (32)P]P(i) into vitellogenin followed a pattern identical with that found for (3)H]leucine in the pulse-labelling experiments and this indicates that synthesis of the polypeptide chain and incorporation of P(i) are closely linked processes. 6. The cumulative evidence suggests that the 2h lag phase represents the time required for the assembly and secretion of this multicomponent protein.