Alpha-actinin-2 is a
protein
Proteins are large biomolecules and macromolecules that comprise one or more long chains of amino acid residues. Proteins perform a vast array of functions within organisms, including catalysing metabolic reactions, DNA replication, res ...
which in humans is encoded by the ''ACTN2''
gene
In biology, the word gene (from , ; "... Wilhelm Johannsen coined the word gene to describe the Mendelian units of heredity..." meaning ''generation'' or ''birth'' or ''gender'') can have several different meanings. The Mendelian gene is a b ...
.
This gene encodes an alpha-actinin isoform that is expressed in both skeletal and cardiac muscles and functions to anchor myofibrillar actin thin filaments and
titin
Titin (contraction for Titan protein) (also called connectin) is a protein that in humans is encoded by the ''TTN'' gene. Titin is a giant protein, greater than 1 µm in length, that functions as a molecular spring that is responsible for th ...
to
Z-disc
A sarcomere (Greek σάρξ ''sarx'' "flesh", μέρος ''meros'' "part") is the smallest functional unit of striated muscle tissue. It is the repeating unit between two Z-lines. Skeletal muscles are composed of tubular muscle cells (called musc ...
s.
Structure
Alpha-actinin-2 is a 103.8 kDa protein composed of 894 amino acids.
Each molecule is rod-shaped (35 nm in length) and it homodimerizes in an anti-parallel fashion. Each monomer has an
N-terminal actin-binding region composed of two calponin homology domains, two
C-terminal
The C-terminus (also known as the carboxyl-terminus, carboxy-terminus, C-terminal tail, C-terminal end, or COOH-terminus) is the end of an amino acid chain (protein or polypeptide), terminated by a free carboxyl group (-COOH). When the protein is ...
EF hand
The EF hand is a helix–loop–helix structural domain or ''motif'' found in a large family of calcium-binding proteins.
The EF-hand motif contains a helix–loop–helix topology, much like the spread thumb and forefinger of the human hand, i ...
domains, and four tandem spectrin-like repeats form the rod domain in the central region of the molecule.
The high-resolution crystal structure of human alpha-actinin 2 at 3.5 Å was recently resolved. Alpha
actinin
Actinin is a microfilament protein. Alpha-actinin-1 is necessary for the attachment of actin myofilaments to the Z-lines in skeletal muscle cells, and to the dense bodies in smooth muscle cells. The functional protein is an anti-parallel dimer, ...
s belong to the
spectrin
Spectrin is a cytoskeletal protein that lines the intracellular side of the plasma membrane in eukaryotic cells. Spectrin forms pentagonal or hexagonal arrangements, forming a scaffold and playing an important role in maintenance of plasma membr ...
gene superfamily which represents a diverse group of actin-binding
cytoskeletal
The cytoskeleton is a complex, dynamic network of interlinking protein filaments present in the cytoplasm of all cells, including those of bacteria and archaea. In eukaryotes, it extends from the cell nucleus to the cell membrane and is compo ...
proteins, including
spectrin
Spectrin is a cytoskeletal protein that lines the intracellular side of the plasma membrane in eukaryotic cells. Spectrin forms pentagonal or hexagonal arrangements, forming a scaffold and playing an important role in maintenance of plasma membr ...
,
dystrophin
Dystrophin is a rod-shaped cytoplasmic protein, and a vital part of a protein complex that connects the cytoskeleton of a muscle fiber to the surrounding extracellular matrix through the cell membrane. This complex is variously known as the cost ...
,
utrophin
Utrophin is a protein that in humans is encoded by the ''UTRN'' gene.
The protein encoded by this gene is a component of the cytoskeleton. Utrophin was found during research into Duchenne's muscular dystrophy. The name is a contraction for ''ubiq ...
and
fimbrin
Fimbrin also known as is plastin 1 is a protein that in humans is encoded by the PLS1 gene. Fimbrin is an actin cross-linking protein important in the formation of filopodia.
Structure
Fimbrin belongs to the calponin homology (CH) domain supe ...
.
Skeletal, cardiac, and smooth muscle isoforms are localized to the
Z-disc
A sarcomere (Greek σάρξ ''sarx'' "flesh", μέρος ''meros'' "part") is the smallest functional unit of striated muscle tissue. It is the repeating unit between two Z-lines. Skeletal muscles are composed of tubular muscle cells (called musc ...
and analogous dense bodies, where they help anchor the myofibrillar actin filaments. Alpha-actinin 2 has been shown to
interact
Advocates for Informed Choice, dba interACT or interACT Advocates for Intersex Youth, is a 501(c)(3) nonprofit organization using innovative strategies to advocate for the legal and human rights of children with intersex traits. The organizati ...
with
KCNA5
Potassium voltage-gated channel, shaker-related subfamily, member 5, also known as KCNA5 or Kv1.5, is a protein that in humans is encoded by the ''KCNA5'' gene.
Function
Potassium channels represent the most complex class of voltage-gated ion ...
,
DLG1
Discs large homolog 1 (DLG1), also known as synapse-associated protein 97 or SAP97, is a scaffold protein that in humans is encoded by the ''SAP97'' gene.
SAP97 is a mammalian MAGUK-family member protein that is similar to the Drosophila protein ...
,
[ ]DISC1
Disrupted in schizophrenia 1 is a protein that in humans is encoded by the ''DISC1'' gene. In coordination with a wide array of interacting partners, DISC1 has been shown to participate in the regulation of cell proliferation, differentiation, m ...
,ADAM12
Disintegrin and metalloproteinase domain-containing protein 12 (previously Meltrin) is an enzyme that in humans is encoded by the ''ADAM12'' gene. ADAM12 has two splice variants: ADAM12-L, the long form, has a transmembrane region and ADAM12-S, a ...
, ACTN3
Alpha-actinin-3, also known as alpha-actinin skeletal muscle isoform 3 or F-actin cross-linking protein, is a protein that in humans is encoded by the ''ACTN3'' gene (named sprinter gene, speed gene or athlete gene) located on chromosome 11. All ...
,MYPN
Myopalladin is a protein that in humans is encoded by the ''MYPN'' gene. Myopalladin is a muscle protein responsible for tethering proteins at the Z-disc and for communicating between the sarcomere and the nucleus in cardiac and skeletal muscle
...
,PDLIM3
Actin-associated LIM protein (ALP), also known as PDZ and LIM domain protein 3 is a protein that in humans is encoded by the ''PDLIM3'' gene. ALP is highly expressed in cardiac and skeletal muscle, where it localizes to Z-discs and intercalated di ...
, PKN, MYOT
Myotilin is a protein that in humans is encoded by the ''MYOT'' gene. Myotilin (myofibrillar titin-like protein) also known as TTID (TiTin Immunoglobulin Domain) is a muscle protein that is found within the Z-disc of sarcomeres.
Structure
Myoti ...
, TTN, NMDAR
The ''N''-methyl-D-aspartate receptor (also known as the NMDA receptor or NMDAR), is a glutamate receptor and ion channel found in neurons. The NMDA receptor is one of three types of ionotropic glutamate receptors, the other two being AMPA and ...
, SYNPO2, LDB3
LIM domain binding 3 (LDB3), also known as Z-band alternatively spliced PDZ-motif (ZASP), is a protein which in humans is encoded by the ''LDB3'' gene. ZASP belongs to the Enigma subfamily of proteins and stabilizes the sarcomere (the basic units ...
, and FATZ.
Function
The primary function of alpha-actinin-2 is to crosslink filamentous actin molecules and titin
Titin (contraction for Titan protein) (also called connectin) is a protein that in humans is encoded by the ''TTN'' gene. Titin is a giant protein, greater than 1 µm in length, that functions as a molecular spring that is responsible for th ...
molecules from adjoining sarcomeres at Z-discs, a function that is modulated by phospholipids. It is clear from studies by Hampton et al. that this crosslinking can assume a variety of conformations, with preferences for 60° and 120° angles. Alpha-actinin-2 also functions in docking signalling molecules at Z-discs, and additional studies have also implicated alpha-actinin-2 in the binding of cardiac ion channels, Kv1.5 in particular.
Clinical significance
Mutations in ''ACTN2'' are associated with hypertrophic cardiomyopathy, as well as dilated cardiomyopathy
Dilated cardiomyopathy (DCM) is a condition in which the heart becomes enlarged and cannot pump blood effectively. Symptoms vary from none to feeling tired, leg swelling, and shortness of breath. It may also result in chest pain or fainting. Co ...
and endocardial fibroelastosis. The diverse functions of alpha-actinin-2 are reflected in the diverse clinical presentation of patients carrying ''ACTN2'' mutations.
References
Further reading
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External links
Mass spectrometry characterization of human ACTN2 at COPaKB
GeneReviews/NIH/NCBI/UW entry on Familial Hypertrophic Cardiomyopathy Overview
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{{Cytoskeletal Proteins
EF-hand-containing proteins