The role of actin nucleators and nucleation promoting factors in nuclear gene expression via MRTF

Actin dynamics play a critical role for regulating nuclear gene expression via the MRTF-SRF transcription factor module. The dissociation of MRTF from an inhibitory complex with G-actin is the fundamental regulatory step. This process also occurs inside the nucleus and is closely connected to actin treadmilling. An involvement of actin nucleators and nucleation promoting factors has been shown, but it is unclear where and how they precisely act on the G-actin:MRTF complex. Our preliminary results show that WH2 domains, which are frequently found in nucleation promoting factors, can indeed activate MRTF-SRF mediated transcription. Furthermore, we found in an unbiased SILAC mass-spec screen that the Arp2/3 complex shows elevated binding to actin when MRTF is released and activated. We therefore propose to investigate the involvement of nucleation promoting factors, their WH2 domains, and the Arp2/3 complex in dissociating the G-actin:MRTF complex, especially inside the nucleus. In parallel, we aim to get new insight on nuclear actin assemblies. We expect to characterise the molecular mechanism of G-actin:MRTF dissociation and to clarify its connection to actin nucleation inside and outside the nucleus.