Postassembly cleavage of a retroviral glycoprotein cytoplasmic domain removes a necessary incorporation signal and activates fusion activity.

Viral protease-mediated cleavage within the cytoplasmic domain of the transmembrane (TM) glycoprotein of the type D retrovirus, Mason-Pfizer monkey virus, removes approximately 16 amino acids from the carboxy terminus of the protein. To determine the functional significance of this cleavage in the virus life cycle, we introduced premature stop codons into the TM coding domain, resulting in the production of truncated glycoproteins. Progressive truncated of the cytoplasmic domain identified the carboxy-terminal third as being required for efficient incorporation of the glycoprotein complex into budding virions and profoundly increased the fusogenic capability of the TM glycoprotein. These results, together with the ability of matrix protein mutations to suppress TM cleavage, imply that this portion of the glycoprotein interacts specifically with the capsid proteins during budding, suppressing glycoprotein fusion function until virus maturation has occurred.