Myosin I and II are members of a superfamily of actin-activated
molecular motors that convert chemical energy into mechanical
work. Most members of this superfamily do not form filaments.
Myosin I is the best studied of these "unconventional" myosins.
We discovered a nuclear isoform of myosin I that contains an
unique 16 amino acid NH2-terminal extension. Light and electron
microscopy revealed that nuclear myosin I co-localizes with RNA
polymerase I and II and biochemical and cell biology studies
established that nuclear myosin I is essential for transcription
by both polymerases. Myosin molecules work in concert with actin
and actin is also abundant in the nucleus. We have shown that
actin is necessary for pre-initiation complex (PIC) formation,
apparently because it facilitates the integration of the RNA
polymerase II complex into the developing PIC. Interestingly,
NMI is not needed for forming PICs, but it does appear to be
necessary for forming the first few bonds during mRNA synthesis.
Other experiments