Nuclear Myosin VI Stabilizes RNA Polymerase II in Transcription Factories

Abstract

While it is widely accepted there is temporal regulation of gene expression, more recently there has been increase studies of spatial regulation. Indeed, transcription is known to occur within discrete foci throughout the nucleus with RNA Polymerase II arranged in hubs or factories. The factories bring together several genes and arrange the chromatin to enable simultaneous gene expression. The hubs are estimated to contain 4-30 RNAPII molecules in a region of 40-200 nm. Despite this quantitative knowledge, the mechanism of hub assembly, organisation and stability remains unknown. To this end, we have used advanced single molecule techniques to visualise RNAPII hubs and track their dynamics in live cells. Following our previous work on nuclear myosin VI, we know myosin-actin interactions are critical for RNAPII transcription. Using STORM imaging, we found a decrease in RNAPII hubs when myosin VI or actin was perturbed. Moreover, using 3D single molecule tracking we found an increase in RNAPII mobility when myosin VI was knocked-down or inhibited. We conclude that myosin VI is critical for the RNAPII hubs to enable gene expression whereby the loss of myosin VI activity leads to a disruption of RNAPII organisation. Lastly, we show how myosin VI uses a load-induced anchoring ability to stabilise RNAPII within the hubs as transcription occurs. In this manner, myosin VI uses its force-sensing properties to regulate gene expression. For the first time, we reveal how the properties of myosin are specifically required for transcription.