Evolutionary and Structural Analysis of SH3BGR/AIP5 family proteins and their interactions with actin

Regulated actin filament assembly and stabilization are crucial for diverse biological processes, including cell motility, endocytosis, phagocytosis, cell adhesion, and intracellular traffic. Each filament in a cellular actin network has a fast growing ‘barbed’ end and a slower growing ‘pointed’ end. While the barbed ends of actin filaments have received great attention for many years, and dozens of different regulators of barbed end actin dynamics have been identified, comparatively less is known about the pointed ends of actin filaments and their regulators. Recently, a new family of pointed end actin regulators has been identified, the SH3BGR/AIP5 protein family, which use a thioredoxin-related (TRX) domain to bind directly to two actin subunits at their pointed ends. In 2023, a Cryo-EM study identified vertebrate SH3BGRL2 at the pointed ends of actin filaments isolated from red blood cells. In 2025, a study from our lab identified the yeast protein Aip5 as having a structurally-related TRX domain, which Aip5 uses to promote actin nucleation (in collaboration with formins) and to cap pointed ends of filaments in vitro and in vivo. In 2026, an in vitro study showed that purified vertebrate SH3BGR proteins each weakly nucleate actin assembly and cap pointed ends. Collectively, these findings have revealed a previously underappreciated role for SH3BGR/AIP5 proteins in controlling actin dynamics and in defining an evolutionarily conserved new family of pointed end regulators. In this thesis, I have used computational approaches to better understand the structural basis of how the TRX domain binds actin, and addressed the accessibility of its proline-rich motif hypothesized to bind SH3 and EVH1 domains, from which the family name was derived. Finally, I cloned and purified three of the vertebrate SH3BGR proteins for future characterization of their pointed end affinities and collaborations with suggested in vivo binding partners.