Alpha Actinin: Definition, Structure, Types, Functions (2023)

Alpha-actinin is an actin-binding protein with F-actin (filamentous) binding activity but no G-actin (globular) binding activity.

They are ubiquitous in the cytoskeleton of muscle cells (calcium insensitive) and non-muscle cells (calcium sensitive) such as neurons and fibroblasts. It is primarily responsible for the structural component of the cytoskeleton that helps bind and organize actin filaments in cells.They belong to a larger superfamily of proteins called spectrins and are distributed in different parts of cells depending on the type of tissue in which they are present.

Your function includesStabilization of muscle contractile apparatus, modulation of multiple receptor activities, and function as a scaffold connecting various proteins in the cytoskeleton used in cell signaling pathways.The presence of alpha-actinin at multiple subcellular sites, such as B. cell protrusions, cell matrix junctions and stress fiber dense regions in both muscle and non-muscle cells, points to the importance of this protein in linking cytoskeletal components to various proteins present in cells.

Their existence as isoforms is genetic and biochemical in mammals and species, including protists,invertebrates, and birds.Mammalian cells exhibit the highest level of diversity, with four different alpha-actinin-encoding genes producing at least six different protein products, each with their own specific tissue type and expression pattern.

actin parent molecule

Actin molecule is a protein that ismost commonly present in eukaryotic cells. They are highly conserved proteins throughout evolution and are involved in more protein-protein interactions than other studied proteins.

• It is aimportant contribution to the muscle contraction apparatus.Actin filaments are made up of repeating units of the actin proteintwo forms - monomeric G-actin (globular) and polymeric F-actin (filamentous).
• They serve as building blocks of the cytoskeletonfacilitates cell movements ranging from maintaining cell shape to cell motility to regulating transcription.
• Actin filaments and myosin are activated during muscle contraction, and tropomyosin proteins regulate their fusion during contraction.They regulate the dynamic structures of different cell types via cellular and subcellular movements.

Alpha-Actinine functions

Their main function includes theCross-linking of actin filaments and other components of the cytoskeleton that affect cohesion and the mechanics of cell movement.The scaffold provides stability to actin filaments and acts as a bridge for integration of the cytoskeleton into signaling pathways. Alpha-actinin is believed to interact with approximately 30 cellular components and their isoforms appear to be active in nuclear events.They are highly active in polarized cells, with extensive actin polymerization supporting cell migration and adhesion.You are theprimary cross-links in tension fibers and are abundant in dendritic spines, leading to focal adhesion and neuritic outgrowth.Their interaction causes localization of this protein in cellular compartments such as plasma membranes with transmembrane receptors and membrane lipids.

Alpha-Actinine The type

Four alpha-actin types can be observed that regulate the activity of the cytoskeleton in muscle and non-muscle cells.

1. ACTN1:Coded by the ACTN1 gene and responsible for Z-line attachment to actin myofibrils in skeletal muscle cells. In smooth muscle cells, ACTN1 attaches dense bodies to the actin filaments
2. ACTN2:Coded by the human ACTN2 gene, composed of 894 amino acids and weighing approximately 103.8 kDa, expressed in skeletal and smooth muscle cells. They anchor filamentous actin fragments to Z-discs mediated by phospholipids.
3. ACTN3:Encoded by the ACTN3 gene, also known as the sprinter gene on chromosome 11, expressed exclusively in type II muscle fibers. They are involved in improving athletic performance and muscle injury recovery in elite athletes.
4. ACTN4:Encoded by the ACTN4 gene and involved in cytoskeletal metastatic regulation. Mutations in this actinin lead to renal glomerulosclerosis.

Alpha-Actinine Structure

Alpha-actinin is an antiparallel homodimer that plays a central role in the interconnection of actin filaments in cells. All spectrin superfamilies including alpha-actinin,have an N-terminal actin-binding domain (ABD), spectrin repeats (SR), and a C-terminal calmodulin-like domain (CaM)as a distinguishing feature.

• The ABD region includes twoCalponin homology (CH) domains.
• TheSpectrin Repeats (SR)are responsible forDetermining the length and flexibility of the actin-binding protein and the nature of the actin filament crosslinks.Because the SR region is cylindrical, it forms the rod domain, and ABD and four SR form the functional core rod in the antiparallel dimer of alpha-actinin.
• The C-terminal CaM domain is located at the end of the central rod opposite the ABD.It has two pairs of EF-hand motifs (EF1/2 and EF3/4) that have a regulatory function in the binding protein.Some actinins bind to the calcium, preventing actin binding at high calcium concentrations in muscle and non-muscle cells, thus allowing actin filaments to crosslink into bundles.

N-terminal actin binding domain (ABD)

ABD of alpha-actinins is the most highly conserved domain within the spectrin family of proteins during the evolutionary conservation of actin, its binding partner.ABD consists of two tandem pairs of calponin homology (CH) consisting of 100 amino acids arranged in four alpha helices.There are two types - type 1 (CH1) and type 2 (CH2), which differ in amino acid sequence. In an unbound state, ABD is assumed to exist in a closed conformational state.

• The core of the actin-binding domain includesfour main helices named A, C, E and G,present in every single calponin (CH) domain.
• Helices A and E are external while helices C and G are sandwiched between them.
• In the spectrin family ABD, three smaller helices can be identified, but only two helices,B and F, are seen in alpha actinin ABD.
• The interaction of the CH1 and CH2 domains is semipolar,with some hydrophobic parts and some polar parts that stabilize the interface.

Studies have shown that the surfaces of the two CH domains complement each other similarly to the interfaces in permanent complexes and complexes with enzyme inhibitors. That will be awardedCH1 and CH2 together result in higher affinity actin binding.It is observed that the CH2 tandem domain alone is not sufficient to crosslink actin filaments, while CH1 has lower binding affinity.

Rod domain multiple spectrin repeats

Because of the diversity in the number of spectrin repeats of the alpha-actinin protein in different organisms, it is considered to be the least conserved domain throughout evolution. You are part of the determinationProtein length and flexibility and the nature of actin filament crosslinks.The central rod region typically has four consecutive spectrin repeats in all known vertebrate alpha-actinin.

• Organisms such as protozoa, fungi, and parasites have one, two, and five repeats, respectively.
• The four repeats in vertebrates are thought to originate from a one-repeat precursor protein through two intragenic duplication events.

Each spectrin repeat has the conformation of a helically wound bundle as a three-dimensional structure.

• An antiparallel homodimer that is theThe alpha-actinin rod domain has a total length of 240 Å and a width of 40-50 Å.
• The repeating units are connected by short, rigid helical linkers that provide structural rigidity to the subunit and dimer, a property necessary for its main function—bundling of actin filaments.

The spectrin repeats, due to their natural propensity for helical secondary structure, have sequences that are mostly basic in nature.

• These reps are usually close80 amino acids longand are characterized by specific sequence motifs, e.gKonsensussequenz Asp-Pro-Gly-X-X-Pro.
• However, the amino acid composition of spectrin repeats differs slightly between different spectrin isoforms and organisms.
• Generally,Spectrin repeats are rich in hydrophobic amino acids such as leucine, isoleucine and valine, which are known to be crucial for the stability of the protein.
• Besides herhave significant levels of proline and glycine. These two amino acids are commonly found in structural proteins and are believed to be critical in maintaining the protein's flexibility and shape.

C-terminal calmodulin-like (CaM) domain

A calcium-binding protein that plays a crucial role in intracellular signaling.The CaM-like domain is often found at the C-terminus (the end of the protein).It is responsible for binding to CaM and transmitting signalsResponse to changes in intracellular calcium levels.Four alpha helicestwo pairs of EF motifs (EF1/2 and EF3/4)are bundled together and folded into a small compact spherical structure to form the CaM domain.

• EF hands are used to bind calcium inside cells are helix-loop-helix motifs.EF hands typically exist in pairs, forming a spherical domain that can coordinate up to two calcium ions.
• Calcium binding causes the globular domain to change significantly from a closed to an open state, rearrangement of the a-helices andexposing hydrophobic residues to the protein surface,Allowing the protein to interact with specific targets.
• EF-hand motifs show functional divergence between alpha-actinin isoforms (muscle and non-muscle).
• Muscle isoforms are regulated by binding of the phospholipid phosphatidylinositol-4,5-biphosphate (PiP2).The EF hands of non-muscle actinin isoforms (such as fibroblasts and neurons) bind calcium and exert regulatory control over the actin-binding activity of neighboring ABD and other proteins.

Due to changes in residues involved in calcium coordination, muscle isoforms have lost their ability to bind calcium.While the calcium-insensitive isoforms of alpha-actinin emerged with the development of muscle tissue, where regulation of actin binding should be independent of calcium flux, the original gene encoding alpha-actinin encoded a calcium-sensitive isoform during the evolution of a-actinin.

Regulation by different mechanisms

Numerous processes control alpha-actin activity, such aspost-transcriptional changes, protein-protein interactions and adaptations to intracellular signaling pathways.For example,Phosphorylation with proteins such as talin and vinculin can alter the activity of alpha-actinins and regulate how they bind to actin filaments. Alpha-actinins can also be controlled by fluctuations in intracellular calcium levels, which can affect the binding of alpha-actinins to actin filaments.Two of the regulatory events are discussed below:

phosphorylation

Phosphorylation adds a phosphate group to a protein that can alter its activity and interaction with other proteins.

• Phosphorylation of specific sites in alpha-actinin can lead to its activation by promoting its association with actin filaments.
• In contrast, other phosphorylation sites can lead to its inhibition by reducing its association with actin filaments.
• The kinase enzymes that catalyze the phosphorylation reaction and the specific sites that become phosphorylated are key factors in determining the effects of phosphorylation on alpha-actinin activity.

An autosomal dominant disease FSGS (Focal Segmental Glomerulosclerosis) caused by the natural mutation of the amino acid sequence in ABD of alpha-actinin 4 (ACTN4) can be modulated by phosphorylation. When tyrosine 4 and tyrosine 31 are phosphorylated via growth factors in hierarchical order, a significant decrease in actin-binding activity of ACTN4 is observed. This prevents excessive cell aggregation and facilitates cell motility.

proteolytic cleavage

Regulation of alpha-actinin protein by proteolytic cleavage mechanismhelps stabilize and organize actin filaments in cells. Proteolytic cleavage results in the formation of smaller alpha-actin fragments that represent different functions and activities than the entire protein.The effects of proteolytic cleavage on alpha-actinin activity are largely determined by the specific proteases that mediate the cleavage and the specific sites that are cleaved.

• Alpha actinin 4 (ACTN4) is responsible for the assembly of actin bundles in cells for focal adhesion and cell motility.
• ACTN4 contains a calpain cleavage site in the N-terminal and C-terminal regions. Cleavage at the N-terminal Tyr13 and Gly14 of ACTN4 does not affect the protein's actin-binding activity, but supports actin bundle assembly involved in cell migration.
• However, cleavage at the C-terminus of ACTN4 or another alpha-actinin can lead to a decrease in calcium-regulated binding of actin filaments, thereby impeding focal adhesion and cell motility.

Calpain protease has also been observed to regulate T cell alpha-actinin activity. Stimulation of CD3 and T-cell receptors activates actinin-mediated rearrangement of the cytoskeleton. Cleaved actinin improves cytoskeletal spreading and plasticity due to facilitation of movement of smaller actinin proteins.

Diploma

The role of alpha-actinin as a multitasking actin-binding crosslinking protein. They are responsible for bundling actin and cytoskeletal filaments and help translocate the membrane and intracellular proteins involved in signaling pathways.

references

1. Sjöblom, Björn, A. Salmazo, and K. Djinović-Carugo. "Structure and regulation of α-actinin." Cellular and Molecular Life Sciences 65 (2008): 2688-2701.
2. Kelkar, Manasi, Peter Bohec, and William Charras. "Mechanics of the cellular actin cortex: From signaling to shape change." Current Opinion in Cell Biology 66 (2020): 69-7
3. Alpha-Actin | MBInfo – https://www.mechanobio.info/cytoskeleton-dynamics/actin-crosslinking/alpha-actinin/
4. Dominguez, Roberto, and Kenneth C. Holmes. "Structure and Function of Actin." Annual Review of Biophysics 40 (2011): 169-186.
5. Actin – https://www.britannica.com/science/actin
6. Ribeiro Jr., Almeida, et al. "The structure and regulation of human muscle α-actinin." (2014).
7. Calponin homology domain - https://en.wikipedia.org/wiki/Calponin_homology_domain
8. Fukami, Kiyoko et al. "Alpha-actinin and vinculin are PIP2-binding proteins involved in tyrosine kinase signaling." Journal of Biological Chemistry 269.2 (1994): 1518-1522.
9. Shao, Hanshuang et al. "The carboxyl tail of alpha-actinin-4 regulates its susceptibility to m-calpain and thus acts in cell migration and spreading." The international journal of biochemistry & cell biology 45.6 (2013): 1051-1063.
10. Shao, Hanshuang et al. "Focal segmental glomerulosclerosis ACTN4 mutants that bind to actin: regulation by phosphomimetic mutations." Scientific Reports 9.1 (2019): 15517.
11. MacArthur, Daniel G. and Kathryn N. North. "A gene for speed? Evolution and function of α‐actinin‐3.” Bioessays 26.7 (2004): 786-795.
12. Selliah, Nithianandan, William H. Brooks, and Thomas L. Roszman. "Proteolytic cleavage of alpha-actinin by calpain in T cells stimulated with anti-CD3 monoclonal antibodies." Journal of Immunology (Baltimore, Md.: 1950) 156.9 (1996): 3215-3221.
13. Actinin – https://en.wikipedia.org/wiki/Actinin
14. Alpha-Actinin-2 – https://en.wikipedia.org/wiki/Alpha-Actinin-2
15. Pickering, Craig, and John Kiely. "ACTN3: more than just a gene for speed." Frontiers in Physiology 8 (2017): 1080.
16. Alpha-Actinin-3 – https://en.wikipedia.org/wiki/Alpha-Actinin-3
17. Alpha-Actinin-4 – https://en.wikipedia.org/wiki/Alpha-Actinin-4