LOCALIZATION OF THE LABILE DISULFIDE BOND BETWEEN SU AND TM OF THE MURINE LEUKEMIA-VIRUS ENVELOPE PROTEIN COMPLEX TO A HIGHLY CONSERVED CWLC MOTIF IN SU THAT RESEMBLES THE ACTIVE-SITE SEQUENCE OF THIOL-DISULFIDE EXCHANGE ENZYMES

Previous studies have indicated that the surface (SU) and transmembran e (TM) subunits of the envelope protein (Env) of murine leukemia virus es (MuLVs) are joined by a labile disulfide bond that can be stabilize d by treatment of virions with thiol-specific reagents. In the present study this observation was extended to the Envs of additional classes of MuLV, and the cysteines of SU involved in this linkage were mapped by proteolytic fragmentation analyses to the CWLC sequence present at the beginning of the C-terminal domain of SU. This sequence is highly conserved across a broad range of distantly related retroviruses and resembles the CXXC motif present at the active site of thiol-disulfide exchange enzymes. A model is proposed in which rearrangements of the SU-TM intersubunit disulfide linkage, mediated by the CWLC sequence, p lay roles in the assembly and function of the Env complex.