Growth and stability of a cholesterol-independent Semliki Forest virus mutant in mosquitoes

Semliki Forest virus (SFV) is an enveloped alphavirus that is transmitted i n the wild by mosquito vectors. In tissue culture cells, SFV requires chole sterol in the cell membrane both for virus membrane fusion and for the effi cient exit of progeny virus from the cell. A previously isolated SN mutant, srf-3, is strikingly less cholesterol-dependent for virus fusion, exit, an d growth due to a single amino acid change in the E1 spike protein subunit, proline 226 to serine. Here we show that when mosquitoes were infected by intrathoracic injection at a range of virus multiplicities, the growth of s rf-3 was significantly more rapid than that of wild-type virus, particularl y at low multiplicity infection. The differential cholesterol requirements for wild-type and srf-3 infection were maintained during virus passage thro ugh mosquitoes. The presence or absence of cholesterol in the srf-3 virus m embrane did not affect its infection properties in mosquitoes. Thus the srf -3 mutation causes a growth advantage in the tissues of the mosquito host ( C) 1999 Academic Press.