Dual Regulatory Role of Nuclear FNBP4 in Actin Binding and Formin FMN1 Inhibition

Nuclear actin dynamics are increasingly recognized as central to genome regulation, yet the identity and function of dedicated nuclear actin regulators remain elusive. Here, we uncover formin-binding protein 4 (FNBP4) as a previously uncharacterized nuclear actin-binding protein that integrates dual regulatory functions. We identify a conserved basic KRRK motif within its C-terminal that directly interacts actin and coincides with a non-canonical nuclear localization signal. In parallel, FNBP4 interacts with the formin FMN1 via its N-terminal WW domains, potently suppressing FMN1-mediated actin nucleation. Strikingly, full-length FNBP4, which unites both the FMN1- and actin-binding modules, displays synergistic inhibition far exceeding that of truncated fragments. Furthermore, confocal imaging confirms nuclear colocalization of FNBP4 with actin, highlighting its role in shaping nuclear actin architecture. Our findings position FNBP4 as the first example of a nuclear-resident formin inhibitor that simultaneously binds actin and a formin. This dual functionality establishes a new regulatory paradigm in which a single protein integrates direct actin interaction with formin inhibition to fine-tune nuclear actin organization.