<Li> There is a structure in subdomain 2 that is called the "D - loop" because it binds with DNase I, it is located between the His40 and Gly48 residues . It has the appearance of a disorderly element in the majority of crystals, but it looks like a β - sheet when it is complexed with DNase I. It has been proposed that the key event in polymerization is probably the propagation of a conformational change from the centre of the bond with the nucleotide to this domain, which changes from a loop to a spiral . However, this hypothesis has been refuted by other studies . </Li> <P> The classical description of F - actin states that it has a filamentous structure that can be considered to be a single stranded levorotatory helix with a rotation of 166 ° around the helical axis and an axial translation of 27.5 Å, or a single stranded dextrorotatory helix with a cross over spacing of 350 - 380 Å, with each actin surrounded by four more . The symmetry of the actin polymer at 2.17 subunits per turn of a helix is incompatible with the formation of crystals, which is only possible with a symmetry of exactly 2, 3, 4 or 6 subunits per turn . Therefore, models have to be constructed that explain these anomalies using data from electron microscopy, cryo - electron microscopy, crystallization of dimers in different positions and diffraction of X-rays . It should be pointed out that it is not correct to talk of a "structure" for a molecule as dynamic as the actin filament . In reality we talk of distinct structural states, in these the measurement of axial translation remains constant at 27.5 Å while the subunit rotation data shows considerable variability, with displacements of up to 10% from its optimum position commonly seen . Some proteins, such as cofilin appear to increase the angle of turn, but again this could be interpreted as the establishment of different structural states . These could be important in the polymerization process . </P> <P> There is less agreement regarding measurements of the turn radius and filament thickness: while the first models assigned a longitude of 25 Å, current X-ray diffraction data, backed up by cryo - electron microscopy suggests a longitude of 23.7 Å . These studies have shown the precise contact points between monomers . Some are formed with units of the same chain, between the "barbed" end on one monomer and the "pointed" end of the next one . 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 a filament is formed by monomers in a "sheet" formation, in which the subdomains turn about themselves, this form is also found in the bacterial actin homologue MreB . </P> <P> The F - actin polymer is considered to have structural polarity due to the fact that all the microfilament's subunits point towards the same end . This gives rise to a naming convention: the end that possesses an actin subunit that has its ATP binding site exposed is called the "(-) end", while the opposite end where the cleft is directed at a different adjacent monomer is called the "(+) end". The terms "pointed" and "barbed" referring to the two ends of the microfilaments derive from their appearance under transmission electron microscopy when samples are examined following a preparation technique called "decoration". This method consists of the addition of myosin S1 fragments to tissue that has been fixed with tannic acid . This myosin forms polar bonds with actin monomers, giving rise to a configuration that looks like arrows with feather fletchings along its shaft, where the shaft is the actin and the fletchings are the myosin . Following this logic, the end of the microfilament that does not have any protruding myosin is called the point of the arrow (- end) and the other end is called the barbed end (+ end). A S1 fragment is composed of the head and neck domains of myosin II . Under physiological conditions, G - actin (the monomer form) is transformed to F - actin (the polymer form) by ATP, where the role of ATP is essential . </P>

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