Mutations of conserved cysteine residues in the CWLC motif of the oncoretrovirus SU protein affect maturation and translocation

The envelope glycoprotein complex is composed of two polypeptides, an external heavily glycosylated polypeptide (SU) and a membrane-spanning protein (TM). Together they form a heterodimer on the surface of the virion. These proteins are synthesized in the form of a polyprotein precursor which is glycosylated and proteolytically processed during its maturation in the secretory pathway. A highly conserved stretch of four amino acids, CWLC, has been identified in most known oncoretroviral SU proteins, about two-thirds of the distance from the amino terminus. To study the significance of this sequence for the structure and/or function of SU, cysteine to serine mutations were made in reticuloendotheliosis virus strain A. Initial studies showed that substitution of either one or both cysteines resulted in the production of noninfectious virus. Furthermore, immunoprecipitations and pulse-chase analysis demonstrated that the mutants yielded envelope polyprotein precursors which were stable. However, the polyprotein precursors were not proteolytically processed into SU and TM, and immunoprecipitations indicate that the immature polyproteins form aggregates, suggesting that the mutations interfere with proper folding. Although not proteolytically processed, at least one of the mutant glycoproteins appeared to be efficiently transported to the cell surface. These studies indicate that changing either cysteine residue abrogates viral infectivity by affecting folding, inhibiting normal maturation of the envelope glycoproteins.