N-Terminal Actin-Binding Site of Lmod2 Promotes Controlled Pointed End Elongation

BACKGROUND: Lmods (leiomodins) are critical for the assembly and maintenance of thin filaments in striated muscles by allowing thin filament elongation at the pointed ends. Lmod2's elongation function has been linked to both actin-binding sites (ABSs) 2 and 3, while the existence and function of an N-terminal ABS1 has been debated. METHODS: To elucidate the little-known role of Lmod2's ABS1, we created a mutant (F64D/L69D/W72D/W73D: Lmod2-quadruple mutant) predicted to decrease the binding of ABS1 to actin. We analyzed the effect of the mutations using several in vitro, cellular, and in vivo assays. RESULTS: By disrupting the interaction of Lmod2 ABS1 with actin in isolated cardiomyocytes and in mice, we engineered a super Lmod2 that results in remarkably longer thin filaments. Structural analysis determined that ABS1 of Lmod2 binds to actin through a disordered region and an amphipathic α-helix. Analysis of the mutated ABS1 revealed that the helix is destroyed, and binding to actin is maintained only in the N-terminal disordered region of Lmod2 ABS1. CONCLUSIONS: These discoveries support a model of controlled thin filament pointed end elongation by Lmod2 and provide the first direct evidence of, as well as the structural and functional mechanistic basis for, Lmod2's physiological leaky cap activity.