ACTN3 (actinin alpha 3; HGNC:165, NCBI Gene ID 89) is a human locus mapped to chromosome band 11q13.2 that encodes antiparallel homodimeric Z-disc actin-crosslinking protein Alpha-Actinin-3 (901 aa; ~100 kDa). ACTN3 acts in fast-twitch type IIx/IIa sarcomeric Z-disc force transmission & calcineurin/glycogen phosphorylase signaling, together with Actin Filament, Titin Giant Sarcomeric Spring, and Myosin Motor Protein. Encodes the fast-twitch sarcomeric Z-disc protein alpha-actinin-3; the common R577X nonsense polymorphism influences sprint power versus oxidative muscle metabolism.
Approved Symbol
ACTN3
Chromosomal Locus
11q13.2
Encoded Product
Antiparallel homodimeric Z-disc actin-crosslinking protein Alpha-Actinin-3 (901 aa; ~100 kDa)
Primary Pathway
Fast-twitch type IIx/IIa sarcomeric Z-disc force transmission & calcineurin/glycogen phosphorylase signaling
Clinical / Therapeutic Role
Common p.Arg577Ter (R577X) polymorphism: 577RR/RX enriched in Olympic sprint/power athletes; 577XX in ~1.5 billion humans shifts muscle toward oxidative endurance & cold tolerance
Gene Structure & Expression
Genomic Organization & Cytogenetic Locus: 21-exon gene on chromosome 11q13.2 encoding a 901-amino-acid spectrin-superfamily structural protein composed of an N-terminal actin-binding domain (two calponin-homology domains), a central rod domain of four spectrin-like repeats that mediates antiparallel homodimerization, and a C-terminal calmodulin-like EF-hand domain.
Cellular & Tissue Expression Profile: Expressed strictly in fast-twitch glycolytic (type IIx) and fast-oxidative-glycolytic (type IIa) skeletal muscle fibers—where it anchors sarcomeric thin actin filaments and titin at the Z-disc—in contrast to its ubiquitous paralog alpha-actinin-2 (ACTN2), which is expressed in all skeletal and cardiac muscle fibers.
Molecular Function & Pathways
Biochemical & Physiological Mechanism: Stabilizes the fast-twitch sarcomeric Z-disc lattice against high-velocity eccentric and explosive contractile forces, modulates glycogen phosphorylase activity, and sequesters the calcineurin-activating protein calsarcin-2 (MYOZ1) so that fast fibers resist calcineurin-driven conversion toward slow oxidative phenotypes.
Clinical & Therapeutic Significance
Human Genetic Variants & Disease Phenotypes: The common nonsense polymorphism rs1815739 (c.1729C>T, p.Arg577Ter or R577X) underwent positive natural selection as modern humans migrated out of Africa into colder Eurasian latitudes: ~18% of the global population (~1.5 billion people) are homozygous 577XX and completely lack alpha-actinin-3 in their fast-twitch fibers, which is compensated structurally by ACTN2 up-regulation, increased calsarcin-2-free calcineurin signaling, higher mitochondrial oxidative enzyme activity, and improved non-shivering cold tolerance via tonic Ca2+ leak, whereas functional 577R alleles are virtually universal among elite Olympic 100-meter sprinters and power athletes.
Therapeutic & Clinical Pharmacology: Serves as a premier human genetic modifier of exercise physiology, age-related sarcopenia (577XX carriers show lower baseline fast-twitch muscle mass), and Duchenne muscular dystrophy progression.