Ca2+-independent binding of an EF-hand domain to a novel motif in the α-actinin-titin complex

The interaction between alpha -actinin and titin, two modular muscle proteins, is essential for sarcomere assembly. We have solved the solution structure of a complex between the calcium-insensitive C-terminal EF-hand domain of alpha -actinin-2 and the seventh Z-repeat of titin. The structure of the complex is in a semi-open conformation and closely resembles that of myosin light chains in their complexes with heavy chain IQ motifs. However, no IQ motif is present in the Z-repeat, suggesting that the semi-open conformation is a general structural solution for calcium-independent recognition of EF-hand domains.