The interaction of titin and alpha-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.