The growth and release of several alphaviruses, including several strains of Sindbis virus (the wild-type strain, the large plaque and small plaque variants of the HR strain, and the HR mutant 103), Semliki Forest virus (SFV) and Middelburg virus, and of the unrelated rhabdovirus, vesicular stomatitis virus (VSV), have been compared in chick cells and in BHK-21 cells as a function of the culture conditions for the host cell and the ionic strength of the medium. The small plaque strain of Sindbis HR, as well as SFV, grew better in BHK cells, whereas the large plaque strain of Sindbis HR showed a preference for chick cells. Wild-type Sindbis and VSV grew equally well in either cell. The optimum ionic strength for virus production as well as inhibition of virus release into the medium at low ionic strength depended upon both the virus and the host cell. Thus, VSV grown in medium of low ionic strength gave no additional release of virus on incubation with hypertonic medium (minimum effect), whereas 103 released very little virus without exposure to hypertonic conditions (maximum effect). The viruses could be ordered as follows: minimum effect = vesicular stomatitis virus < Middelburg virus < Semliki Forest virus < Sindbis wt < Sindbis HR (large plaque) < Sindbis HR (small plaque) < Sindbis 103 = maximum effect. After several passages in culture, chick cells required hypertonic conditions for optimum production and release of Sindbis virus. Furthermore, BHK cells cultured in different media responded differently to ionic strength for virus production and release. These results suggest that there is a charge-dependent step in the maturation of alpha-viruses, possibly a configurational rearrangement of glycoprotein E2 upon its formation from the precursor PE2, which is sensitive to the ionic strength of the medium, to the composition of the host plasmalemma and to differences in the virus glycoproteins.


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