The SH3BGRL protein family nucleates and caps actin filaments via its conserved thioredoxin fold

Cellular actin polymerization is a tightly regulated process, typically controlled by proteins with specialized domains such as the Wiskott-Aldrich syndrome protein homology 2 (WH2) domain. Here, we identify SH3BGRL family proteins as modulators of actin dynamics, uniquely characterized by their thioredoxin (Trx) fold structure and the absence of the canonical CXXC enzymatic site essential for redox activity. The Trx fold is generally associated with enzymatic activity; however, in this context, it functions non-enzymatically to enhance actin filament nucleation and inhibit depolymerization. The family member SH3BGRL-2 was previously identified as part of the spectrin-actin complex in porcine erythrocytes. Further structural analysis reveals that human SH3BGRL proteins share structural homology with the C-terminal region of Saccharomyces cerevisiae YFR016c/Aip5, a known actin nucleation factor reported to bind G-actin. Notably, our results show that human SH3BGRL proteins do not bind G-actin directly. While they do not interact with G-actin, SH3BGRL proteins significantly increase actin assembly rates by accelerating filament nucleation without affecting barbed end elongation, as demonstrated in pyrene-actin bulk-polymerization assays and total internal reflection fluorescence microscopy (TIRFM) based single-filament studies. Furthermore, using all-atom molecular dynamics (MD) simulations and in vitro assays that directly probe the pointed end of the actin filament, we show that SH3BGRL proteins inhibit the depolymerization of existing filaments by interacting with the pointed end of the actin filament, also in the presence of the well-characterized pointed end capping protein tropomodulin. Our results indicate that all SH3BGRL family proteins promote actin nucleation by stabilizing energetically unstable actin dimers and trimers and inhibit depolymerization by direct association with the pointed end, suggesting a direct role for the Trx fold in actin dynamics.