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 mo lecules requires developmental and spatial regulation of protein interactio ns. In muscle, titin acts as a molecular ruler organizing the actin cytoske leton via interactions with many sarcomeric proteins, including the crossli nking 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. alp ha -actinin is a rod shaped head-to-tail homodimer. In contrast to C-termin al fragments, full-length alpha -actinin does not bind Z-repeats. We identi fy a 30-residue Z-repeat homologous sequence between the actin-binding and rod regions of alpha -actinin that binds the C-terminal domain with nanomol ar affinity. Thus, Z-repeat binding is prevented by this 'pseudoligand' int eraction between the subunits of the alpha -actinin dimer. This autoinhibit ion is relieved upon binding of the Z-disk lipid phosphatidylinositol-bisph osphate to the actin-binding domain. We suggest that this novel mechanism i s relevant to control the site-specific interactions of alpha -actinin duri ng sarcomere assembly and turnover. The intramolecular contacts defined her e also constrain a structural model for intrasterical regulation of all alp ha -actinin isoforms.