Phosphorylation and dephosphorylation events play critical roles in Sindbis virus maturation

We have examined the effects of various inhibitors of protein kinases and phosphatases on Sindbis virus maturation in BHK cells. 2-aminopurine, a nonspecific protein kinase inhibitor, N-(6-aminohexyl)-5-chloro-1-naphthalenesulfonamide hydrochloride (W-7), a specific inhibitor of calmodulin/Ca(2+)-dependent protein kinase, and okadaic acid (OKA), a protein phosphatase inhibitor, dose-dependently inhibited Sindbis virus maturation. Although virus production was inhibited, the membrane glycoprotein precursors PE2/E1 were exported from the endoplasmic reticulum and PE2 was converted to E2 at normal kinetic rates. The glycoproteins were delivered to the plasma membrane in conformations which rendered them competent for low pH-mediated cell-cell fusion from within. Electron microscopy showed that in the presence of W-7, virus nucleocapsids were free in the cell cytoplasm, while in the presence of OKA, the nucleocapsids were associated with cell membranes. Metabolic labeling of Sindbis virus-infected cells with [32P]orthophosphate in the presence of OKA resulted in the specific labeling of the PE2/E2 glycoprotein. We have previously shown that the carboxyl terminus of the PE2 glycoprotein is initially buried in cell membranes and is then exposed to the cytoplasm at some later stage in virus maturation. The data shown are consistent with the hypothesis that phosphorylation and dephosphorylation play a critical role in a late stage in Sindbis virus maturation, possibly in releasing of the E2 tail from cell membranes.