PE2 cleavage mutants of Sindbis virus: Correlation between viral infectivity and pH-dependent membrane fusion activation of the spike heterodimer

The spike glycoprotein E2 of Sindbis virus (SIN) is synthesized in the infe cted cell as a PE2 precursor protein, which matures through cleavage by a c ellular furin-like protease. Previous work has shown that SIN mutants impai red in PE2 cleavage are noninfectious on BHK-21 cells, the block in infecti on being localized at a step after virus-receptor interaction but prior to RNA replication. Here, we studied the membrane fusion properties of SIN PE2 cleavage mutants and observed that these viruses are impaired in their abi lity to form an El homotrimer and to fuse with liposomes at a mildly acidic pH. The block in spike rearrangement and fusion could be overridden by exp osure of the mutant viruses to very low pH (<4.5). Cleavage mutants with se cond-site resuscitating mutations in PE2 were highly infectious for BHK-21 cells. The ability of these viruses to form El homotrimers and to fuse at a mildly acidic pH was completely restored despite a sustained lack of PE2 c leavage.