THE TRANSMEMBRANE DOMAIN IN VIRAL FUSION - ESSENTIAL ROLE FOR A CONSERVED GLYCINE RESIDUE IN VESICULAR STOMATITIS-VIRUS G-PROTEIN

The transmembrane (TM) domains of viral fusion proteins are required f or fusion, but their precise role is unknown, G protein, the fusion pr otein of vesicular stomatitis virus, was previously shown to lose sync ytia-forming ability if six residues (GLIIGL) were deleted from its TM domain, The 20-residue TM domain of wild-type (TM20) G protein was th us changed into a TM domain of 14 residues (TM14), To assess possible sequence specificity for this loss of function, the two Gly residues i n TM20 were replaced with either Ala or Leu, Both mutations resulted i n complete loss of fusion activity, as measured by fusion-dependent re porter gene transfer, Single substitutions decreased activity by about half, TM14 was weakly active (15%) but reintroduction of a Gly residu e into TM14 by a single Ile --> Gly substitution increased activity to 80%. All mutants retained normal hemifusion activity, i.e., lipid mix ing between the outer leaflets of the reacting membranes, Thus, at lea st one TM Gly residue is required for a late step in fusion mediated b y G protein, Gly residues were significantly (2.6-fold; P = 0.004) mor e abundant in the TM domains of viral fusion proteins than in those of nonfusion proteins and were distributed differently within the TM dom ain, Thus, Gly residues in the TM domain of other viral fusion protein s may also prove to be important for fusion activity.