<Ul> <Li> The adenosine nucleotide binding site is located between two beta hairpin - shaped structures pertaining to the I and III domains . The residues that are involved are Asp11 - Lys18 and Asp154 - His161 respectively . </Li> <Li> The divalent cation binding site is located just below that for the adenosine nucleotide . In vivo it is most often formed by Mg or Ca while in vitro it is formed by a chelating structure made up of Lys18 and two oxygens from the nucleotide's α - and β - phosphates . This calcium is coordinated with six water molecules that are retained by the amino acids Asp11, Asp154, and Gln137 . They form a complex with the nucleotide that restricts the movements of the so - called "hinge" region, located between residues 137 and 144 . This maintains the native form of the protein until its withdrawal denatures the actin monomer . This region is also important because it determines whether the protein's cleft is in the "open" or "closed" conformation . </Li> <Li> It is highly likely that there are at least three other centres with a lesser affinity (intermediate) and still others with a low affinity for divalent cations . It has been suggested that these centres may play a role in the polymerization of actin by acting during the activation stage . </Li> <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. Domínguez "et al ." suggest 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> </Ul> <Li> The adenosine nucleotide binding site is located between two beta hairpin - shaped structures pertaining to the I and III domains . The residues that are involved are Asp11 - Lys18 and Asp154 - His161 respectively . </Li> <Li> The divalent cation binding site is located just below that for the adenosine nucleotide . In vivo it is most often formed by Mg or Ca while in vitro it is formed by a chelating structure made up of Lys18 and two oxygens from the nucleotide's α - and β - phosphates . This calcium is coordinated with six water molecules that are retained by the amino acids Asp11, Asp154, and Gln137 . They form a complex with the nucleotide that restricts the movements of the so - called "hinge" region, located between residues 137 and 144 . This maintains the native form of the protein until its withdrawal denatures the actin monomer . This region is also important because it determines whether the protein's cleft is in the "open" or "closed" conformation . </Li> <Li> It is highly likely that there are at least three other centres with a lesser affinity (intermediate) and still others with a low affinity for divalent cations . It has been suggested that these centres may play a role in the polymerization of actin by acting during the activation stage . </Li>

Where would actin be synthesized in a cell