Proteolytic activation of respiratory syncytial virus fusion protein - Cleavage at two furin consensus sequences

The F (fusion) protein of the respiratory syncytial viruses is synthesized as an inactive precursor F-0 that is proteolytically processed at the multi basic sequence KKRKRR136 into the subunits F-1 and F-2 by the cellular prot ease furin. This maturation process is essential for the F protein to gain fusion competence. We observed that proteolytic cleavage additionally occur s at another basic motif, RARR(109), that also meets the requirements for f urin recognition. Cleavage at both sites leads to the removal from. the pol ypeptide chain of a glycosylated peptide of 27 amino acids. When the sequen ce RARR(109) was changed to NANR(109) or to RANN(109) by site-directed muta genesis, cleavage by furin was completely prevented. Although the mutants w ere still processed at position Arg(136). they did not show any syncytia fo rmation. Proteolytic cleavage of the modified motifs was achieved by treatm ent of transfected cells with trypsin converting the F mutants into their f usogenic forms. Our findings indicate that both furin consensus sequences h ave to be cleaved in order to activate the fusion protein.