Disulfide bonds are essential for the stability of the Sindbis virus envelope

Sindbis virus is a membrane-containing virus which has two glycoproteins organized in an icosahedral lattice. Protein-protein associations have been identified which participate in the formation of the icosahedron and these associations are stabilized by intramolecular disulfide bridges (Anthony, R. P., and Brown, D. T., 1990, J. Virol. 65, 1187-1194). The present study further examines the role of disulfides in the structure and function of Sindbis virus by following the effect of dithiothreitol on the protease sensitivity of envelope proteins as well as the electron microscopic appearance and infectivity of Sindbis virus. Treatment of isolated virus with 5 mM dithiothreitol for 6 hr causes a marked increase in trypsin sensitivity of both E1 and E2, profound morphological alterations in the viral envelope, increased susceptibility of the nucleocapsid to RNase, and 95% loss of infectivity. These effects are greatly enhanced and accelerated when treatment with DTT is preceded by a brief exposure of the virus to pH 5.3, suggesting that acid-induced conformational changes render structurally critical disulfides more accessible to reductive cleavage by DTT. When compared to other manipulations known to change the conformation of the viral envelope, such as heating to 51 or 60 degrees or exposure to acid pH, only the exposure to DTT with or without prior acid treatment caused marked structural changes correlated with a loss of infectivity. These data provide electron microscopic and functional evidence that intact disulfide bonds are critical for the stability of the virus envelope and suggest that the cleavage of critical disulfide(s) may play a role in the process of virus infection.