Cooperative cleavage of the R peptide in the Env trimer of Moloney murine leukemia virus facilitates its maturation for fusion competence

The spike protein of murine leukemia virus, MLV, is made as a trimer of the Env precursor. This is primed for receptor-induced activation of its membrane fusion function first by cellular furin cleavage in the ectodomain and then by viral protease cleavage in the endodomain. The first cleavage separates the peripheral surface (SU) subunit from the transmembrane (TM) subunit, and the latter releases a 16-residue-long peptide (R) from the TM endodomain. Here, we have studied the distribution of R peptide cleavages in the spike TM subunits of Moloney MLV preparations with partially R-peptide-processed spikes. The spikes were solubilized as trimers and separated with an R peptide antibody. This showed that the spikes were either uncleaved or cleaved in all of its TM subunits. Further studies showed that R peptide cleavage-inhibited Env mutants, L(649)V and L(649)I, were rescued by wild-type (wt) Env in heterotrimeric spikes. These findings suggested that the R peptide cleavages in the spike are facilitated through positive allosteric cooperativity; i.e., the cleavage of the TM subunit in one Env promoted the cleavages of the TMs in the other Envs. The mechanism ensures that protease cleavage in newly released virus will generate R-peptide-cleaved homotrimers rather than heterotrimeric intermediates. However, using a cleavage site Env mutant, L(649)R, which was not rescued by wt Env, it was possible to produce virus with heterotrimers. These were shown to be less fusion active than the R-peptide-cleaved homotrimers. Therefore, the cooperative cleavage will speed up the maturation of released virus for fusion competence.