摘要：

The four major internal structural proteins (the group-specific antigens) of avian myeloblastosis virus are formed by sequential cleavage of a precursor polypeptide with M r = 76,000 (Pr76). The evidence for this conclusion is based on analysis of immune precipitates from lysates of AMV § § Abbreviation used: AMV, avian myeloblastosis virus.-infected cells treated with a multivalent antiserum directed against these proteins. Sodium dodecyl sulfate gel electrophoresis of such immune precipitates from cells pulse-labeled with [ 35S]-methionine reveals five metabolically unstable radioactive polypeptides. These polypeptides behave kinetically as precursors to virion proteins. Double-label ion-exchange chromatography of tryptic digests of the unstable polypeptides demonstrates that the largest precursor, Pr76, contains the amino acid sequences of all four virion proteins. It appears not to contain the sequence of the fifth and smallest internal virion protein. The four smaller precursors are intermediate cleavage products of Pr76. The arrangement of the virion proteins in Pr76 was determined by labeling cells shortly after inhibiting polypeptide chain initiation. The relative amounts of radioactivity both in completed virion proteins and in the tryptic peptides of Pr76 implies the same order for three of the four proteins. The exact position of one protein remains uncertain. On the basis of these experiments, we propose a cleavage pathway for the generation of the structural proteins of AMV. We also demonstrate that cleavage of precursors can proceed in crude extracts of AMV-infected cells. This proteolysis, while resistant to several protease inhibitors, is completely blocked by addition of agents that disrupt membranes.

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