UCLA

Actin and myosin form contractile structures that drive muscle contraction, cell motility, cytokinesis, and many other cellular functions. These functions depend crucially on the correct assembly, organization, and maintenance of actin filaments. Formins are a major class of actin nucleators that assemble many actin-based structures through their formin homology (FH) 1 and 2 domains, which together both nucleate and processively elongate actin filaments. The formin homology domain-containing protein (Fhod) family of formins is responsible for the assembly of several contractile actin structures, including sarcomeres in muscle cells and stress fibers in many non-muscle cells. Although Fhod family members are required for actin assembly in vivo, mammalian Fhods were reported to instead inhibit actin assembly in vitro. Here, we establish that Drosophila Fhod, human Fhod1, and human Fhod3 all nucleate and elongate actin filaments in vitro. We show that the nucleation activity of Fhod1 depends on the actin isoform, with a significantly weaker ability to nucleate skeletal muscle actin compared to other isoforms. Drosophila and human Fhod proteins are weak elongators, with slow elongation rates and short run lengths compared to other formins. Elongation by Drosophila Fhod is impaired by a coiled coil in the N-terminus, which constrains the ability of the FH1 domain to capture profilin-actin, and a disordered C-terminal tail, which promotes dissociation of Fhod from growing actin filaments. Finally, we establish human stem cell-derived cardiomyocytes as a model system to investigate how Fhod proteins assemble and organize actin in the sarcomere. Taken together, this work establishes a framework to understand the mechanisms by which the Fhod family of formins assembles contractile actin structures.