The
matrix (M)
protein of
vesicular stomatitis virus (VSV) is a potent inhibitor in vivo of transcription by all three host
RNA polymerases (RNAP). In the case of host
RNA polymerase II (RNAPII), the inhibition is due to lack of activity of the
TATA-binding protein (
TBP), which is a subunit of the basal
transcription factor TFIID. Despite the potency of M
protein-induced inhibition in vivo, experiments presented here show that M
protein cannot directly inactivate
TFIID in vitro. Addition of M
protein to nuclear extracts from uninfected cells did not inhibit transcription activity, indicating that the inhibition is indirect and is mediated through host factors. The host factors that are known to regulate
TBP activity include phosphorylation by host
kinases and association with different
TBP-associated factor (TAF) subunits. However,
TBP in VSV-infected cells was found to be assembled normally with its TAF subunits, as shown by ion exchange high-pressure liquid chromatography and sedimentation velocity analysis. A normal pattern of phosphorylation of
TBP in VSV-infected cells was also observed by pH gradient gel electrophoresis. Collectively, these data indicate that M
protein inactivates
TBP activity in RNAPII-dependent transcription by a novel mechanism, since the known mechanisms for regulating
TBP activity cannot account for the inhibition.