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

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Abstract
The process by which the egg-yolk protein precursor vitellogenin is biosynthesized, assembled and secreted by Xenopus laevis (South African clawed toad) liver was studied. It was previously shown in other laboratories that vitellogenin contains the 2 egg-yolk proteins lipovitellin (MW 140,000) and phosvitin (MW 35,000). Evidence is presented which shows that Xenopus liver microsomal fractions synthesize precursors of vitellogenin. These precursors were solubilized from the membranes with detergent and analyzed by sodium dodecyl sulfate/polyacrylamide-gel electrophoresis. This analysis indicated that there is only 1 precursor polypeptide, and this has about 200,000 .+-. 20,000 MW. The egg-yolk proteins are translated as part of this larger polypeptide. Experiments also demonstrate the existence of a microsomal proteinase which is able to cleave the precursor into smaller fragments. The nature of these fragments provided some indirect evidence that phosvitin and lipovitellin light chains are situated together within the precursor molecule. These precursor data fit in well with structural studies on serum vitellogenin, since it has been shown that the latter protein consists of 2 identical subunits each with a mobility on sodium dodecyl sulfate/polyacrylamide gels identical with that shown by the microsomal precursor. Both the intracellular precursor and subunit of vitellogenin have similar (but not necessarily identical) molecular weights. Trypsin or chymotrypsin can cleave the serum vitellogenin into leucine- and serine-rich fragments which resemble lipovitellin and phosvitin, respectively. The serine-rich fragment is not identical with phosvitin, since it contains 8 time more leucine than that expected for the authentic phosvitin molecule.

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