Differential Actin-regulatory Activities of Tropomodulin1 and Tropomodulin3 with Diverse Tropomyosin and Actin Isoforms

Tropomodulins (Tmods) are F-actin pointed end capping proteins that interact with tropomyosins (TMs) and cap TM-coated filaments with higher affinity than TM-free filaments. Here, we tested whether differences in recognition of TM or actin isoforms by Tmod1 and Tmod3 contribute to the distinct cellular functions of these Tmods. We found that Tmod3 bound ∼5-fold more weakly than Tmod1 to α/βTM, TM5b, and TM5NM1. However, surprisingly, Tmod3 was as effective as Tmod1 at capping pointed ends of skeletal muscle α-actin (α_sk-actin) filaments coated with α/βTM, TM5b, or TM5NM1. Tmod3 only capped TM-coated α_sk-actin filaments more weakly than Tmod1 in the presence of recombinant αTM2, which is unacetylated at its NH₂ terminus, binds F-actin weakly, and has a disabled Tmod-binding site. Moreover, both Tmod1 and Tmod3 were similarly effective at capping pointed ends of platelet β/cytoplasmic γ (γ_cyto)-actin filaments coated with TM5NM1. In the absence of TMs, both Tmod1 and Tmod3 had similarly weak abilities to nucleate β/γ_cyto-actin filament assembly, but only Tmod3 could sequester cytoplasmic β- and γ_cyto-actin (but not α_sk-actin) monomers and prevent polymerization under physiological conditions. Thus, differences in TM binding by Tmod1 and Tmod3 do not appear to regulate the abilities of these Tmods to cap TM-α_sk-actin or TM-β/γ_cyto-actin pointed ends and, thus, are unlikely to determine selective co-assembly of Tmod, TM, and actin isoforms in different cell types and cytoskeletal structures. The ability of Tmod3 to sequester β- and γ_cyto-actin (but not α_sk-actin) monomers in the absence of TMs suggests a novel function for Tmod3 in regulating actin remodeling or turnover in cells.