Smooth muscle α-actinin interaction with smitin

Actin-myosin II filament-based contractile structures in striated muscle, smooth muscle, and nonmuscle cells also contain the actin filament-crosslinking protein α-actinin. In striated muscle sarcomeres, interactions between the myosin-binding protein titin and α-actinin in the Z-line provide an important structural linkage. We previously discovered a titin-like protein, smitin, associated with the contractile apparatus of smooth muscle cells. Purified native smooth muscle α-actinin binds with nanomolar affinity to smitin in smitin-myosin coassemblies in vitro. Smooth muscle α-actinin also interacts with striated muscle titin. In contrast to striated muscle α-actinin interaction with titin and smitin, which is significantly enhanced by PIP2, smooth muscle α-actinin interacts with smitin and titin equally well in the presence and absence of PIP2. Using expressed regions of smooth muscle α-actinin, we have demonstrated smitin-binding sites in the smooth muscle α-actinin R2-R3 spectrin-like repeat rod domain and a C-terminal domain formed by cryptic EF-hand structures. These smitin-binding sites are highly homologous to the titin-binding sites of striated muscle α-actinin. Our results suggest that direct interaction between α-actinin and titin or titin-like proteins is a common feature of actin-myosin II contractile structures in striated muscle and smooth muscle cells and that the molecular bases for α-actinin interaction with these proteins are similar, although regulation of these interactions may differ according to tissue.