Regulation of vaccinia virus morphogenesis: phosphorylation of the A14L and A17L membrane proteins and C-terminal truncation of the A17L protein are dependent on the F10L kinase.

This study focused on three vaccinia virus-encoded proteins that participate in early steps of virion morphogenesis: the A17L and A14L membrane proteins and the F10L protein kinase. We found that (i) the A17L protein was cleaved at or near an AGX consensus motif at amino acid 185, thereby removing its acidic C terminus; (ii) the nontruncated form was associated with immature virions, but only the C-terminal truncated protein was present in mature virions; (iii) the nontruncated form of the A17L protein was phosphorylated on serine, threonine, and tyrosine residues, whereas the truncated form was unphosphorylated; (iv) nontruncated and truncated forms of the A17L protein existed in a complex with the A14L membrane protein; (v) C-terminal cleavage of the A17L protein and phosphorylation of the A17L and A14L proteins failed to occur in cells infected with a F10L kinase mutant at the nonpermissive temperature; and (vi) the F10L kinase was the only viral late protein that was necessary for phosphorylation of the A17L protein, whereas additional proteins were needed for C-terminal cleavage. We suggest that phosphorylation of the A17L and A14L proteins is mediated by the F10L kinase and is required to form the membranes associated with immature virions. Removal of phosphates and the A17L acidic C-terminal peptide occur during the transition to mature virions.