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ACTN1
ACTN1 Full Name
actinin, alpha 1
ACTN1 Introduction
ACTN1 encodes α-actinin-1, a non-muscle isoform of the α-actinin family of actin-crosslinking proteins. The gene maps to chromosome 14q24.1 and spans approximately 105 kb. α-Actinin-1 functions as an antiparallel homodimer in which each monomer is organized into three structural modules: an N-terminal actin-binding domain (ABD) composed of two calponin homology (CH) subdomains that engage F-actin; a central rod domain containing four spectrin-like repeats (SR1–SR4) that mediate head-to-tail dimerization and provide mechanical flexibility; and a C-terminal calmodulin-like domain (CaM) harboring EF-hand motifs that regulate actin affinity. Unlike the muscle isoforms ACTN2 and ACTN3, α-actinin-1 is expressed ubiquitously and is relatively insensitive to calcium, making it particularly suited for the constitutive scaffolding functions required in non-muscle cell types. Within cells, ACTN1 crosslinks actin filaments into stress fibers, localizes to focal adhesions, and links transmembrane integrins to the intracellular actin cytoskeleton, thereby coordinating cell adhesion, spreading, and mechanosensing.
Figure 1. Localisation of α-actinin in skeletal muscle. (Source: Del Coso J, et al. 2019)
In platelets and megakaryocytes, ACTN1 plays a particularly critical role. During megakaryopoiesis, α-actinin-1 participates in the organization of the actin cytoskeleton that drives proplatelet formation — the long cytoplasmic extensions from which platelets are eventually shed into the bloodstream. Within mature platelets, ACTN1 maintains the discoid resting shape and supports cytoskeletal remodeling upon activation, including interaction with integrin αIIbβ3 that is essential for stable thrombus formation and clot retraction. The specificity of ACTN1-related disease for the platelet lineage, despite its ubiquitous expression, is thought to reflect the unique mechanical demands of platelet biogenesis and the limited redundancy afforded by co-expressed paralogs ACTN2 and ACTN4 in the megakaryocyte context.
Heterozygous loss-of-function or dominant-negative mutations in ACTN1 cause autosomal dominant macrothrombocytopenia, also classified as Bleeding Disorder, Platelet-Type 15. This disorder is characterized by a reduced platelet count and variably enlarged platelets, though rod domain variants may produce normal platelet size, complicating diagnosis. Clinically, the condition is generally mild, with most patients discovered incidentally during routine blood counts and showing little to no bleeding tendency; however, a subset of individuals may experience post-surgical bleeding. More than 77 pathogenic variants distributed across the ABD, rod domain, and CaM domain have been catalogued, with missense mutations being the most common. Functional studies have demonstrated that all pathogenic variants disrupt normal actin cytoskeletal organization in vitro, producing thickened, branched actin fibers with approximately 2.5-fold increased fiber area compared to wild-type cells. Collectively, these findings establish ACTN1-RT as one of the most frequent forms of inherited thrombocytopenia, estimated to account for 4–6% of unexplained constitutional thrombocytopenia cases.
Alternate Names for ACTN1
ACTN1; actinin, alpha 1; BDPLT15; alpha-actinin-1; actinin 1 smooth muscle; non-muscle alpha-actinin-1; F-actin cross-linking protein; alpha-actinin cytoskeletal isoform;
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