The interaction of titin and α-actinin is controlled by a phospholipid-regulated intramolecular pseudoligand mechanism

The assembly of stable cytoskeletal structures from dynamically recycled molecules requires developmental and spatial regulation of protein interactions. In muscle, titin acts as a molecular ruler organizing the actin cytoskeleton via interactions with many sarcomeric proteins, including the crosslinking protein alpha -actinin. An interaction between the C-terminal domain of alpha -actinin and titin Z-repeat motifs targets alpha -actinin to the Z-disk. Here we investigate the cellular regulation of this interaction. alpha -actinin is a rod shaped head-to-tail homodimer. In contrast to C-terminal fragments, full-length alpha -actinin does not bind Z-repeats. We identify a 30-residue Z-repeat homologous sequence between the actin-binding and rod regions of alpha -actinin that binds the C-terminal domain with nanomolar affinity. Thus, Z-repeat binding is prevented by this 'pseudoligand' interaction between the subunits of the alpha -actinin dimer. This autoinhibition is relieved upon binding of the Z-disk lipid phosphatidylinositol-bisphosphate to the actin-binding domain. We suggest that this novel mechanism is relevant to control the site-specific interactions of alpha -actinin during sarcomere assembly and turnover. The intramolecular contacts defined here also constrain a structural model for intrasterical regulation of all alpha -actinin isoforms.