Actin

• α-actinin isoform
• β-actinin isoform
• γ-actinin isoform

1. Cell cytoskeleton is formed, creating mechanical support.
2. Involved in myosin-independent cell shape changes and cell movement.
3. In muscle cells, actin together with myosin creates a complex involved in muscle contraction .
4. In non-muscle cells, it participates in the transport of vesicles and organelles by myosin ^[1]
5. Cell division and cytokinesis

Electron microscope images showed that G-actin has a spherical structure; however, x-ray crystallography showed that each of these globules consists of two lobes separated by a groove. This structure is an “ATPase fold,” which is the center of enzymatic catalysis, which binds ATP and Mg ²⁺ , and hydrolyzes the former to ADP and organic phosphate. This fold is a conservative structure, which is also found in other proteins ^[3] . G-actin only functions when it contains either ADP or ATP in its groove, but the form associated with ATP prevails in cells when actin is present in monomeric form ^[4] .

Primary Structure

Contains 374 amino acid residues. Its N-terminus is strongly acidic and begins with acetylated aspartate in its amino group. Although its C-terminus is alkaline and is formed by phenylalanine , which is preceded by cysteine ^[5] .

Tertiary Structure - Domains

The tertiary structure is formed by two domains, known as large and small, which are separated from each other by a groove. Below this is a deeper notch called the “groove”. Both structures have comparable depth ^[6] .

Topological studies have shown that a protein with the largest domain on the left side and the smallest domain on the right side. In this position, the smaller domain, in turn, is divided into two: subdomain I (lower position, residues 1-32, 70-144 and 338-374) and subdomain II (upper position, residues 33-69). The larger domain is also divided into two: subdomain III (lower, residues 145–180 and 270–337) and subdomain IV (higher, residues 181–269). Open areas of subdomains I and III are called “jagged” ends, and exposed areas of domains II and IV are called “pointed” ends.

The classical description of F-actin states that it has a filamentous structure, which can be considered either as a single-chain left spiral with a rotation of 166 ° around the spiral axis and an axial shift of 27.5 Å , or as a single-chain right spiral with a cross spacing of 350-380 Å, moreover each actin molecule is surrounded by 4 others. The symmetry of the actin polymer at 2.17 subunits per helix revolution is incompatible with the formation of crystals, which is possible only with symmetry of exactly 2, 3, 4, or 6 subunits per revolution ^{[7] [8]} .

It is believed that the F-actin polymer has a structural polarity due to the fact that all microfilament subunits point to the same end. This leads to a naming convention: the end that has an actin subunit that has an ATP binding site is called the “(-) end”, while the opposite end, where the cleft is directed to another adjacent monomer, is called the “(+) end "The terms" pointed "and" jagged ", referring to the two ends of microfilaments, derive from their appearance by transmission electron microscopy when the samples are examined using a preparation method called" decoration. "This myosin forms polar bonds with actin mon measures, which leads to a configuration that looks like arrows with perforations along its shaft, where the shaft is actin and flatching is myosin. Following this logic, the end of a microfilament that does not have protruding myosin is called the arrow point (- end), and the other the end is called the prickly end (+ end) ^[9] . The S1 fragment consists of the domains of the head and neck of myosin II. Under physiological conditions, G-actin (monomeric form) is transformed into F-actin (polymer form) using ATP, where the role of ATP is substantial.

The process of forming a polymer actin called F-actin involves the binding of monomeric G-actin to the ATP molecule in the presence of Mg ²⁺ , Ca ^{2+ ions} , the formation of stable actin oligomers and globules, the formation of individual actin polymer filaments and their branching. As a result, organic phosphate and an ADP molecule are formed. Actin microfilaments are formed by spiral twisting of 2 F-actin filaments, inside which actin molecules are linked together by non-covalent bonds ^[10]

Each such microfilament has two ends that differ in their properties: actin monomers are attached to one (it is called the plus-end), and dissociate from the other (minus-end). The ratio of the rates of addition and dissociation of actin monomers determines whether the filament lengthens or shortens ^[10] .