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

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Abstract
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 α‐actinin. An interaction between the C‐terminal domain of α‐actinin and titin Z‐repeat motifs targets α‐actinin to the Z‐disk. Here we investigate the cellular regulation of this interaction. α‐actinin is a rod shaped head‐to‐tail homodimer. In contrast to C‐terminal fragments, full‐length α‐actinin does not bind Z‐repeats. We identify a 30‐residue Z‐repeat homologous sequence between the actin‐binding and rod regions of α‐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 α‐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 α‐actinin during sarcomere assembly and turnover. The intramolecular contacts defined here also constrain a structural model for intrasterical regulation of all α‐actinin isoforms.