Phosphoinositide binding regulates α-actinin dynamics: Mechanism for modulating cytoskeletal remodeling

The active association-dissociation of dynamic protein-protein interactions is critical for the ability of the actin cytoskeleton to remodel. To determine the influence of phosphoinositide binding on the dynamic interaction of α-actinin with actin filaments and integrin adhesion receptors, fluorescence recovery after photobleaching (FRAP) microscopy was carried out comparing wild-type green fluorescent protein (GFP)-α-actinin and a GFP-α-actinin mutant with a decreased affinity for phosphoinositides (Fraley, T. S., Tran, T. C., Corgan, A. M., Nash, C. A., Hao, J., Critchley, D. R., and Greenwood, J. A. (2003) J. Biol. Chem. 278, 24039-24045). In fibroblasts, recovery of the mutant α-actinin protein was 2.2 times slower than the wild type along actin stress fibers and 1.5 times slower within focal adhesions. FRAP was also measured in U87MG glioblastoma cells, which have higher levels of 3-phosphorylated phosphoinositides. As expected, α-actinin turnover for both the stress fiber and focal adhesion populations was faster in U87MG cells compared with fibroblasts with recovery of the mutant protein slower than the wild type along actin stress fibers. To understand the influence of α-actinin turnover on the modulation of the actin cytoskeleton, wild-type or mutant α-actinin was co-expressed with constitutively active phosphoinositide (PI) 3-kinase. Co-expression with the α-actinin mutant inhibited actin reorganization with the appearance of enlarged α-actinin containing focal adhesions. These results demonstrate that the binding of phosphoinositides regulates the association-dissociation rate of α-actinin with actin filaments and integrin adhesion receptors and that the dynamics of α-actinin is important for PI 3-kinase-induced reorganization of the actin cytoskeleton. In conclusion, phosphoinositide regulation of α-actinin dynamics modulates the plasticity of the actin cytoskeleton influencing remodeling.