AlphaActinin

Alpha-actinin is a family of actin-binding proteins that crosslink actin filaments and organize the cytoskeleton. In humans, four vertebrate isoforms are encoded by ACTN1, ACTN2, ACTN3, and ACTN4. ACTN2 and ACTN3 are predominantly expressed in muscle tissue, while ACTN1 and ACTN4 are more widely expressed in non-muscle cells but can also be present in muscle.

Each alpha-actinin protein has an N-terminal actin-binding domain, a central rod domain composed of spectrin-like repeats, and a C-terminal calmodulin-like domain with potential calcium-binding capacity. They form antiparallel dimers and crosslink actin filaments to create networks or bundles, with isoform- and context-dependent effects on filament organization and dynamics.

Localization and function vary by tissue. In striated muscle, alpha-actinins reside at the Z disk and help anchor actin filaments, contributing to sarcomere integrity and force transmission. In non-muscle cells, they localize to focal adhesions, stress fibers, and cell-cell junctions, where they influence cell shape, adhesion, and migration. Alpha-actinins also participate in signaling pathways by interacting with other cytoskeletal and signaling proteins, such as titin and vinculin.

Clinical relevance includes disease associations and population variation. ACTN4 mutations are linked to autosomal dominant focal segmental glomerulosclerosis; ACTN1 mutations have been associated with congenital macrothrombocytopenia. A common ACTN3 variant, R577X, results in loss of functional alpha-actinin-3 and correlates with differences in muscle performance. Altered expression of alpha-actinins has been observed in cancer and may affect cell motility in certain contexts.