Mechanistic Insights into FNBP4-Mediated Regulation of non-diaphanous Formin FMN1 in Actin Cytoskeleton Dynamics

Formin1 (FMN1), a member of the non-diaphanous formin family, is essential for development and neuronal function. Unlike diaphanous-related formins, FMN1 is not subject to canonical autoinhibition through the DID and DAD domains, nor is it activated by Rho GTPase binding. Recent studies suggest that formins also play roles in the nucleus, influencing DNA damage response and transcriptional regulation. However, the mechanisms regulating nuclear formins particularly non-diaphanous ones like FMN1 remain poorly understood. Our previous research identified the interaction between FMN1 and FNBP4, prompting further investigation into its functional role in regulating actin dynamics. Results reveal that FNBP4 inhibits FMN1-mediated actin assembly in vitro. It is shown that FNBP4 prevents FMN1 from displacing the capping protein CapZ at the growing barbed end of actin filaments. Additionally, FNBP4 inhibits FMN1s bundling activity in a concentration-dependent manner. Further analysis indicates that FNBP4 interacts with the FH1 domain and the interdomain connector between the FH1 and FH2 domains, creating spatial constraints on the FH2 domain. We propose that FNBP4 acts as a stationary inhibitor of FMN1. In addition, we identify a monopartite nuclear localization signal (NLS) in FNBP4, and subcellular localization studies show that FNBP4 colocalizes with FMN1. This study provides new insights into the regulatory role of FNBP4 in modulating FMN1-mediated actin dynamics, suggesting that FNBP4 may function as a nuclear inhibitor of actin polymerization and shedding light on regulatory mechanisms specific to non-diaphanous formins.