Structure of the actively growing actin filament pointed end facilitated by Leiomodin-2

Cardiac contraction depends on synchronized interactions between myosin-based thick filaments and actin-based thin filaments (TFs). Precise regulation of TF length is vital for cardiac function, as any alteration in length leads to severe myopathies. Actin filaments form the backbone of the TF and have two unequal ends - fast-growing barbed and slow-growing pointed. In muscle, the barbed end is capped at the Z-line, so the thin filament can elongate only from the pointed end. Leiomodin-2 (Lmod2) promotes actin nucleation and possesses a unique C-terminal extension (CTE) that is important for actin nucleation and binds to the sides of mature TFs. Single-molecule studies have shown that Lmod2 remains bound processively to the growing pointed end. We employed cryo-electron microscopy to visualize the structure of actively growing pointed ends in the presence of Lmod2 from G-actin complexed with profilin (profilactin). We show that Lmod2's leucine-rich repeat domain (LRR) stabilizes terminal actin subunits by binding across the helical groove of actin on one or both actin strands. LRR binding pushes the terminal actins outward from their ideal positions in the actin filament, introducing strain at the pointed end that squeezes Lmod2 from the filament's exterior, thereby maintaining the concentration of Lmod2 required for further elongation. Further, we suggest that the unique Lmod2 CTE may stabilize Lmod2 binding to the actively growing pointed end.