Alpha Actinin- Definition, Structure, Regulations, Functions

The cytoskeleton contains the filamentous system, which includes polymers of actin, intermediate filament protein, and tubulin. The organizational principle of these proteins is that large complex structures are made up of small, simpler components. These filamentous network structures are highly dynamic and stable. These highly organized structures are involved in various functions such as maintaining the cell’s structural scaffold, providing mechanical stability, locomotion, intracellular transport, etc. The dynamic nature of actin filaments, their assembly, and disassembly are responsible for cell movement or migration. Actin filaments are assembled in two structural forms: bundles and networks. Many actin-binding proteins play major roles in the organization, formation, and functioning of the actin cytoskeleton.

Alpha Actinin
Figure: Alpha Actinin. Left: Ca2+-Independent Binding of an EF-Hand Domain to a Novel Motif in the Alpha-Actinin-Titin Complex. Right: The crystal structure of the actin-binding domain from alpha-actinin in its closed conformation: structural insight into phospholipid regulation of alpha-actinin. Created with biorender.com.

Alpha Actinin Definition

Alpha actinin is a conserved protein that crosslinks the actin filament. It belongs to a conserved family of the actin-binding protein, which is the spectrin superfamily, which also includes dystrophin and spectrin. Alpha actinin is present in their isoforms. In mammalian cells,4 alpha-actinin coding genes produce six different protein products, which are found in different tissues with different expressions.

Alpha actinin can be grouped into two different classes on the basis of expression, function, and biochemical characteristics : 

  1. Muscles (which are calcium insensitive)
  2. Non–muscle cytoskeleton (which are muscle sensitive) isoforms. 

The non-muscle isoforms are associated with the focal contact and stress fibers, but their distribution is different in highly motile cells

Alpha Actinin Structure

  • It consists of a rod domain and an actin-binding domain.
  • 100 kD rod-shaped proteins that form head to tail homodimers.
  • Alpha actinin monomer consists of three distinct domains: N- terminal actin-binding domain [ABD], four spectrin-like repeats, and C terminal EF-hands.

ROD domain

  • the central rod region is composed of the spectrin repeat, and it is the least conserved region of the alpha-actinin. 
  • forms an anti-parallel homodimer which has an overall length of 240 angstroms and width of 40 – 50 angstroms.
  •  The repeating units are connected with a rigid and short linker, which provides structural rigidity. 
  • role in the bundling of the actin filaments.
  • 90-degree twisted along the axis of the dimer.
  • Electrostatic potential on the surface is acidic and acts as a docking platform.
  • In skeletal and cardiac muscle: cross-links anti-parallel actin filaments coming from adjacent sarcomeres
  • In non-muscle and smooth muscle: show the diverse orientation.

Actin binding domain

  • highly conserved domain 
  • contain a pair of type 1 and type 2, CH domain, and also found in other types of acting binding proteins.
  • Due to the interdomain interaction of 700-900 angstrom, it forms a closed structure.
  • The core of the domain is formed by the joining of four principle helices (A, C, E, G) that form a single CH domain.
  • The nature of the CH1-CH2 domain is semipolar, in which some portions show hydrophobicity and the rest show polar interactions.
  • The two domains are not functionally equivalent but together show a high affinity for binding with actin filaments.
  • The electron density of this domain is bell-shaped, with a base of 38 angstroms and a height of 42 angstroms.
  • It consists of two calpain homology domains, also termed as CH domain, a neck region, four spectrin repeats [SR], and two calmodulin-like domains.

CaM-like domain

  • The binding of calcium induces a conformational change of a globular domain from closed to open, causing alpha helix rearrangement and hydrophobic residue exposure on the surface to make it accessible to interact with its target.
  • The C-terminus of the CaM-like domain is formed from 4 EF-hand motifs.
  • In non-muscle isoforms: EF binds to calcium and regulates the activity of the ABD domain, thereby reducing the actin-binding property at a certain concentration of calcium.
  • Muscle isoforms: don’t bind with calcium.

Alpha Actinin Regulations

  • The first binding partner of alpha-actinin: Cytoplasmic tails of the beta-subunit of integrin and the intracellular adhesion molecule-1 (ICAM-1)
  • The interaction is between the negatively charged rod domain and positive cytoplasmic peptides.
  • Alpha actinin is a component of dense regions, that is, ‘periodic structures found in stress fibers that are considered to be structural and functional analogs of the sarcomere Z-disk’.
  • It binds to zyxin and CRP, thus acting as a scaffold for interactions and subcellular distribution of proteins.
  • Binds to enigma/cipher family of proteins and possessed N-terminal PDZ domain.
  • In muscle isoforms: interact with TM receptors, contractile machinery, adaptors associated with it, many signaling proteins, etc.
  • For example, an important signaling molecule known as calcineurin in the skeletal muscle acts in the determination of the type of muscle fibers and hypertrophy.
  • Alpha actinin of sacroyces interacts with enzymes involved in metabolization such as phosphorylase in glycogenolysis and fructose-1-6-bisphosphate an aldose.

Alpha Actinin Functions

Apart from its mechanical role, it plays many other important roles in the cell. 

  • It links many transmembrane proteins in a variety of junctions.
  • It also regulates the activity of several receptors and 
  • It serves as a scaffold to connect the cytoskeleton to various signaling pathways.

Role in human kidney disease

  • A mutation in the ACTN 4, which is an alpha-actinin coding gene, causes a rare form of familial focal segmental glomerulosclerosis in humans. 
  • Resulting in mild to severe proteinuria with decreased kidney functioning. 
  • The mutation leads to increased binding affinity to filamentous actin that results in abnormal accumulation of the actin-rich cellular aggregates.
  • There are four human alpha-actinin  [ACTN 1-4]  family members.  However, due to an unclear reason, the human phenotype which is associated with the ACTN – 4 mutation is found only in the kidney. 
  • Two other missense mutations were found in the ABD, W59R, and I149del
    • These were defined to be disease-causing and resulted in the following:
    • Abnormal cellular aggregates of F actin are formed in the cell.
    • These mutations increased the binding affinity of the ACTN – 4 to F actin.
    • Mutation in ACTN 4 co-segregate within the family.
    • The mutation occurs within the ABD domain. 
  • Another clinical report had documented a germline mutation in the  ACTN 4 at S262F in the father of two affected siblings. Both of these patients had developed FSGS in the early stages of life [3-4 years old]. 

References

  1. Feng D, DuMontier C, Pollak MR. The role of alpha-actinin-4 in human kidney disease. Cell Biosci. 2015;5:44. Published 2015 Aug 18. doi:10.1186/s13578-015-0036-8.
  2. Mutations in the gene that encodes the F-actin binding protein anillin cause FSGS.
  3. Rasheed A Gbadegesin et al., J Am Soc Nephrol, 2014.
  4. Alpha-actinin structure and regulation. B. Sjöblom, A. Salmazo, K. Djinović-Carugo Cell Mol Life Sci. 2008 Sep; 65(17): 2688–2701. DOI: 10.1007/s00018-008-8080-8.
  5. Sjöblom, B., Salmazo, A., & Djinović-Carugo, K. (2008). α-Actinin structure and regulation. Cellular and Molecular Life Sciences, 65(17), 2688–2701. doi:10.1007/s00018-008-8080-8.

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