Elastase-mediated Activation of the Severe Acute Respiratory Syndrome Coronavirus Spike Protein at Discrete Sites within the S2 Domain

Proteolytic priming is a common method of controlling the activation of membrane fusion mediated by viral glycoproteins. The severe acute respiratory syndrome coronavirus spike protein (SARS-CoV S) can be primed by a variety of host cell proteases, with proteolytic cleavage occurring both as the S1/S2 boundary and adjacent to a fusion peptide in the S2 domain. Here, we studied the priming of SARS-CoV S by elastase and show an important role for residue Thr(795) in the S2 domain. A series of alanine mutants were generated in the vicinity of the S2 cleavage site, with the goal of examining elastase-mediated cleavage within S2. Both proteolytic cleavage and fusion activation were modulated by altering the cleavage site position. We propose a novel mechanism whereby SARS-CoV fusion protein function can be controlled by spatial regulation of the proteolytic priming site, with important implications for viral pathogenesis.