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 participat e in early steps of virion morphogenesis: the A17L and A14L membrane protei ns and the F10L protein kinase, We found that (i) the A17L protein was clea ved at or near an AGX consensus motif at amino acid 185, thereby removing i ts acidic C terminus; (ii) the nontruncated form was associated with immatu re virions, but only the C-terminal truncated protein was present in mature virions; (iii) the nontruncated form of the A17L protein was phosphorylate d on serine, threonine, and tyrosine residues, whereas the truncated form w as unphosphorylated; (iv) nontruncated and truncated forms of the A17L prot ein existed in a complete with the A14L membrane protein; (v) C-terminal cl eavage of the A17L protein and phosphorylation of the A17L and A14L protein s failed to occur in cells infected with a F10L kinase mutant at the nonper missive temperature; and (vi) the F10L kinase was the only viral late prote in that was necessary for phosphorylation of the A17L protein, whereas addi tional proteins were needed for C-terminal cleavage. We suggest that phosph orylation 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 tran sition to mature virions.