Expression of the human MxA protein is associated with hyperphosphorylation of VSV P protein in human neural Cells

Constitutive expression of the type I interferon-inducible human cytoplasmic MxA protein has been shown to interfere with primary transcription of vesicular stomatitis virus (VSV) in tissue culture cells. As phosphorylation of the VSV P protein has been linked to its ability to stimulate viral transcription, we analyzed the phosphorylation status of this protein in human brain cells (U-87) stably transfected with MxA. We observed a general increase in cellular kinase activity in the presence of MxA, affecting both cellular proteins and VSV P protein. Phosphorylation of the latter was up to threefold higher both in vivo and in vitro. In vitro phosphorylation of recombinant VSV P protein could be enhanced in MxA-negative cell extracts after exogenous addition of recombinant His-MxA. Biochemical evidence and phosphorylation of a mutant P protein lacking the recognized casein kinase II (CKII) sites suggested that hyperphosphorylation of VSV P protein was not due to a stimulation of CKII. We thus propose that expression of MxA in human brain cells is associated with the stimulation of a cellular kinase that is active in phosphorylating both cellular target proteins and VSV P protein.