Actin

Actin is a multifunctional globular protein that forms microfilaments. It is found in almost all eukaryotic cells, where it can be found in concentrations of over 100 Micro molars; its mass is 42 kDa, and its diameter is 4 to 7 nm. Microfilaments, one of the three main components of the cytoskeleton, and thin filaments, which are part of the contractile apparatus in muscle cells, are both made up of actin proteins. Beta actin and alpha filament of actin are the isoforms of actin filament.

Types of Actin

Actin exists in two forms: 

• G-actin (or globular actin) 

• F-actin (or fibrous actin). 

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Let us discuss G actin and F actin in detail

G Actin

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G actin is a single polypeptide chain with a molecular weight of 42 kDa that has a globular shape. Each G actin monomer has one high-affinity calcium-binding site, which helps to keep the molecule's globular shape. Each monomer of G-actin has one ATP binding site. F-actin is a filamentous polymer made up of monomers of G-actin. G-actin appears to have a globular form in scanning electron microscope images, but X-ray crystallography reveals that each of these globules is made up of two lobes separated by a cleft. 

The structure of actin represents the ATPase fold, an enzymatic catalysis centre that binds ATP and Mg2+ and hydrolyzes the latter to ADP and phosphate. This fold can also be present in other proteins that interact with triphosphate nucleotides, such as hexokinase and Hsp70 proteins. When G-actin is present in its free state, it is only functional when it includes either ADP or ATP in its cleft, but when actin is present in its free state, the form that is bound to ATP predominates in cells.

F Actin

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Two helical aggregates of G-actin are twisted around each other in F-actin filaments, with 13.5 subunits per turn. F-actin filaments are the microfilaments used in electron micrographs of cells and can be detected using actin filament-binding compounds or antibodies in immunofluorescence staining procedures. 

F-actin has a filamentous structure that can be thought of as a single-stranded levorotatory helix with a rotation of 166° around the helical axis and an axial translation of 27.5, or a single-stranded dextrorotatory helix with a cross over the spacing of 350–380, with each actin surrounded by four others. The actin polymer's symmetry of 2.17 subunits per helix turn is incompatible with crystal forming, which requires asymmetry of exactly 2, 3, 4, or 6 subunits per helix turn.

The specific contact points between monomers have been revealed in these studies. Some are made up of units from the same chain, sandwiched between one monomer's "barbed" end and the "pointed" end of the next. While the monomers in adjacent chains make lateral contact through projections from subdomain IV, with the most important projections being those formed by the C-terminus and the hydrophobic link formed by three bodies involving residues 39–42, 201–203, and 286. This model suggests that monomers form a filament in a "sheet" shape, in which the subdomains turn around each other; this structure is also found in the bacterial actin homologue MreB.

The structural polarity of the F-actin polymer is due to the fact that all of the microfilament's subunits point in the same direction. The end with an actin subunit with its ATP binding site exposed is referred to as the (-) end, while the opposite end, where the cleft is guided at a separate adjacent monomer, is referred to as the (+) end. 

Tropomyosin, a 40-nanometer long protein wrapped around the F-actin helix, is also present in the helical F-actin filament found in muscles. Tropomyosin protects the active sites of actin during the resting period, preventing the actin-myosin interaction from occurring and causing a muscular contraction. Troponins, which have three polymers: troponin I, troponin T, and troponin C, are other protein molecules that are attached to the tropomyosin thread.

Structure of Actin

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The amino acid sequence of actin is one of the most conserved among proteins, having changed little over time and varying by no more than 20% in organisms as diverse as algae and humans. As a result, it is thought to have a well-designed structure. It has two defining characteristics: it is an enzyme that steadily hydrolyzes ATP, biological processes universal energy currency. ATP, on the other hand, is needed to keep the structure intact. 

An almost unique folding mechanism creates its efficient structure. It can also carry out more interactions than any other protein, allowing it to perform a broader range of functions at almost every stage of cellular life than other proteins. Myosin is an example of an actin-binding protein. Another example is villin, which, depending on the concentration of calcium cations in the surrounding medium, can weave actin into bundles or cut the filaments.

In eukaryotes, actin is one of the most abundant proteins, found throughout the cytoplasm. Muscle fibres account for 20% of total cellular protein by weight, while other cells account for between 1% and 5%. There is more than one form of actin, and the genes that code for it are classified as a gene family. This means that each person's genetic material includes instructions for producing actin variants (called isoforms) with slightly different functions. 

This indicates that eukaryotic organisms express a variety of genes that produce -actin, which is found in contractile structures, it is found at the expanding edge of cells that move by projecting their cellular structures, it is also found in the filaments of stress fibres. There is evolutionary conservation in the structure and function also between species in various eukaryotic domains, in addition to the similarities that occur between an organism's isoforms. MreB, a protein capable of polymerizing into microfilaments, has been identified as an actin homologue in bacteria, and Ta0583, an actin homologue in archaea, is even more similar to eukaryotic actins.

G-actin (monomeric globules) and F-actin (polymeric filaments) are the two types of cellular actin. Microfilament is another term for F-actin. To lie correctly on top of each other, two parallel F-actin strands must rotate 166 degrees. The cytoskeleton's microfilaments have a double helix arrangement as a result of this. Microfilaments have a diameter of around 7 nm and a helix that repeats every 37 nm. Each molecule of actin is linked to a Mg2+ cation by a molecule of adenosine triphosphate (ATP) or adenosine diphosphate (ADP). 

Working Mechanism

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Actin is an ATPase, which means it is an ATP hydrolyzing enzyme. The slow reaction rates of this group of enzymes distinguish them. This ATPase is considered to be "active," meaning that when actin becomes part of a filament, its speed increases by 40,000 times. Under ideal conditions, this rate of hydrolysis is estimated to be about 0.3 s1. The Pi is then bound to the actin next to the ADP for a long time before being cooperatively released from the filament's interior.

The precise molecular features of the catalytic mechanism are still unknown. Although there is much debate on this topic, it appears clear that a "closed" conformation is necessary for ATP hydrolysis, and that the residues involved in the process travel to the proper distance. Subdomain 1 contains the glutamic acid Glu137, which is one of the primary residues. Its job is to bind the water molecule that causes a nucleophilic attack on the -phosphate bond in ATP, while the nucleotide is tightly bound to subdomains 3 and 4. Because of the broad distance and distorted position of the water molecule in relation to the reactant, the catalytic process is slow. 

The conformational change caused by the rotation of the domains between actin's G and F types is very likely to bring Glu137 closer to the surface, allowing it to be hydrolyzed. According to this model, the polymerization and ATPase functions will be decoupled immediately. In molecular dynamics and QM/MM simulations, the "open" to "closed" transition between G and F types, as well as its effects on the relative motion of several main residues and the formation of water wires, has been studied.

Function of Actin Filament

• Actin filaments are important components of the eukaryotic cytoskeleton, capable of rapid polymerization and depolymerization. Actin filaments form larger-scale networks in most cells, which are needed for a variety of cell functions.

• Actin networks provide mechanical support to cells as well as trafficking routes through the cytoplasm that aid signal transduction. The ability of cells to migrate is allowed by the rapid assembly and disassembly of the actin network (Cell migration).

• Non conventional myosins use ATP hydrolysis to transport cargo including vesicles and organelles much faster than diffusion. The arrangement of myosin allows myosin V to function as an effective cargo export motor and myosin VI to function as an effective cargo import motor.

• Actin is a protein that can be present in both the cytoplasm and the nucleus of a cell. Its position is regulated by cell membrane signal transduction pathways, which incorporate the stimuli that a cell receives and cause actin networks to reorganise in response. 

• Phospholipase D has been discovered to interfere with inositol phosphate pathways in Dictyostelium. In muscle fibres, actin filaments are especially stable and plentiful. Actin is found in both the I and A bands of the sarcomere (the essential morphological and physiological unit of muscle fibres); myosin is also found in the latter.

Did You Know?

• Alpha actin is present along actin filaments and in adhesion sites in non-muscle cells.

• ACTA2 (actin alpha 2), also known as alpha-actin, alpha-actin-2, aortic smooth muscle, or alpha-smooth muscle actin, is an actin protein with many aliases.

• In humans, there are six distinct actin isoforms. Beta actin is one of them.