<p>ATPases (or ATP synthases) are membrane-bound enzyme complexes/ion transporters that combine ATP synthesis and/or hydrolysis with the transport of protons across a membrane. ATPases can harness the energy from a proton gradient, using the flux of ions across the membrane via the ATPase proton channel to drive the synthesis of ATP. Some ATPases work in reverse, using the energy from the hydrolysis of ATP to create a proton gradient. There are different types of ATPases, which can differ in function (ATP synthesis and/or hydrolysis), structure (e.g., F-, V- and A-ATPases, which contain rotary motors) and in the type of ions they transport [<cite idref="PUB00020603"/>, <cite idref="PUB00020604"/>]. The different types include:</p><p> <ul><li>F-ATPases (F1F0-ATPases), which are found in mitochondria, chloroplasts and bacterial plasma membranes where they are the prime producers of ATP, using the proton gradient generated by oxidative phosphorylation (mitochondria) or photosynthesis (chloroplasts).</li><li>V-ATPases (V1V0-ATPases), which are primarily found in eukaryotic vacuoles and catalyse ATP hydrolysis to transport solutes and lower pH in organelles.</li><li>A-ATPases (A1A0-ATPases), which are found in Archaea and function like F-ATPases (though with respect to their structure and some inhibitor responses, A-ATPases are more closely related to the V-ATPases).</li><li>P-ATPases (E1E2-ATPases), which are found in bacteria and in eukaryotic plasma membranes and organelles, and function to transport a variety of different ions across membranes.</li><li>E-ATPases, which are cell-surface enzymes that hydrolyse a range of NTPs, including extracellular ATP.</li> </ul> </p><p>P-ATPases (sometime known as E1-E2 ATPases) (<db_xref db="EC" dbkey="3.6.3.-"/>) are found in bacteria and in a number of eukaryotic plasma membranes and organelles [<cite idref="PUB00009616"/>]. P-ATPases function to transport a variety of different compounds, including ions and phospholipids, across a membrane using ATP hydrolysis for energy. There are many different classes of P-ATPases, each of which transports a specific type of ion: H<sup>+</sup>, Na<sup>+</sup>, K<sup>+</sup>, Mg<sup>2+</sup>, Ca<sup>2+</sup>, Ag<sup>+</sup> and Ag<sup>2+</sup>, Zn<sup>2+</sup>, Co<sup>2+</sup>, Pb<sup>2+</sup>, Ni<sup>2+</sup>, Cd<sup>2+</sup>, Cu<sup>+</sup> and Cu<sup>2+</sup>. P-ATPases can be composed of one or two polypeptides, and can usually assume two main conformations called E1 and E2.</p><p>This entry represents the actuator (A) domain, and some transmembrane helices found in P-type ATPases [<cite idref="PUB00002805"/>]. It contains the TGES-loop which is essential for the metal ion binding which results in tight association between the A and P (phosphorylation) domains [<cite idref="PUB00038122"/>]. It does not contain the phosphorylation site. It is thought that the large movement of the actuator domain, which is transmitted to the transmembrane helices, is essential to the long distance coupling between formation/decomposition of the acyl phosphate in the cytoplasmic P-domainand the changes in the ion-binding sites buried deep in themembranous region [<cite idref="PUB00055017"/>]. This domain has a modulatory effect on the phosphoenzyme processing steps through its nucleotide binding [<cite idref="PUB00050553"/>],[<cite idref="PUB00039927"/>].</p><p> P-type (or E1-E2-type) ATPases that form an aspartyl phosphate intermediate in the course of ATP hydrolysis, can be divided into 4 major groups [<cite idref="PUB00003431"/>]: (1) Ca<sup>2+</sup>-transporting ATPases; (2) Na<sup>+</sup>/K<sup>+</sup>- and gastric H<sup>+</sup>/K<sup>+</sup>-transporting ATPases; (3) plasma membrane H<sup>+</sup>-transporting ATPases (proton pumps) of plants, fungi and lower eukaryotes; and (4) all bacterial P-type ATPases, except the g<sup>2+</sup>-ATPase of <taxon tax_id="602">Salmonella typhimurium</taxon>, which is more similar to the eukaryotic sequences. However, great variety of sequence analysis methods results in diversity of classification.</p><p>More information about this protein can be found at Protein of the Month: ATP Synthases [<cite idref="PUB00020719"/>].</p>