Plasma membrane Ca2+ ATPase
The plasma membrane Ca2+ ATPase (PMCA) is a transport protein in the plasma membrane of cells and functions to remove calcium (Ca2+) from the cell. PMCA function is vital for regulating the amount of Ca2+ within all eukaryotic cells. There is a very large transmembrane electrochemical gradient of Ca2+ driving the entry of the ion into cells, yet it is very important that they maintain low concentrations of Ca2+ for proper cell signalling. Thus, it is necessary for cells to employ ion pumps to remove the Ca2+. The PMCA and the sodium calcium exchanger (NCX) are together the main regulators of intracellular Ca2+ concentrations. Since it transports Ca2+ into the extracellular space, the PMCA is also an important regulator of the calcium concentration in the extracellular space.
The pump is powered by the hydrolysis of adenosine triphosphate (ATP), with a stoichiometry of one Ca2+ ion removed for each molecule of ATP hydrolysed. It binds tightly to Ca2+ ions (has a high affinity, with a Km of 100 to 200 nM) but does not remove Ca2+ at a very fast rate. This is in contrast to the NCX, which has a low affinity and a high capacity. Thus, the PMCA is effective at binding Ca2+ even when its concentrations within the cell are very low, so it is suited for maintaining Ca2+ at its normally very low levels. Calcium is an important second messenger, so its levels must be kept low in cells to prevent noise and keep signalling accurate. The NCX is better suited for removing large amounts of Ca2+ quickly, as is needed in neurons after an action potential. Thus the activities of the two types of pump complement each other.
Ca2+/calmodulin binds and further activates the PMCA, increasing the affinity of the protein's Ca2+-binding site 20 to 30 times. Calmodulin also increases the rate at which the pump extrudes Ca2+ from the cell, possibly up to tenfold.
In brain tissue, it has been postulated that certain types of PMCA are important for regulating synaptic activity, since the PMCA is involved in regulating the amount of calcium within the cell at the synapse, and Ca2+ is involved in release of synaptic vesicles. Additionally, it has been shown that PMCA activity is modulated and partly powered by glycolysis in neuronal somata and dendrites. Presumably, it is due to PMCA proximity to glucose transporters in the plasma membrane.
The structure of the PMCA is similar to that of the SERCA calcium pumps, which are responsible for removing calcium from the cytoplasm into the lumen of the sarcoplasmic reticulum. It is thought that the PMCA pump has 10 segments that cross the plasma membrane, with both C and N termini on the inside of the cell. At the C terminus, there is a long "tail" of between 70 and 200 amino acids in length. This tail is thought to be responsible for regulation of the pump.
Each isoform is coded by a different gene and is expressed in different areas of the body. Alternate splicing of the mRNA transcripts of these genes results in different subtypes of these isoforms. Over 20 splice variants have been identified so far.
Three PMCA isoforms, PMCA1, PMCA2, and PMCA3, occur in the brain in varying distributions. PMCA1 is ubiquitous throughout all tissues in humans, and without it embryos do not survive. Lack of PMCA4, which is also very common in many tissues, is survivable, but leads to infertility in males. PMCA types 2 and 3 are activated more quickly and are, therefore, better suited to excitable cell types such as those in nervous and muscle tissue, which experiences large influxes of Ca2+ when excited. PMCA types 1, 2, and 4 have been found in glial cells called astrocytes in mammals, though it was previously thought that only the NCX was present in glia. Astrocytes help to maintain ionic balance in the extracellular space in the brain.
When the PMCA fails to function properly, disease can result. Improperly functioning PMCA proteins have been found associated with conditions such as sensorineural deafness, diabetes, and hypertension.
In excitotoxicity, a process in which excessive amounts of the neurotransmitter glutamate overactivate neurons, resulting in excessive influx of Ca2+ into cells, the activity of the PMCA may be insufficient to remove the excess Ca2+.
In breast tissue, mammary epithelial cells express PMCA2, which transports calcium across the apical surface of the cells into milk. PMCA2 expression falls on weaning, leading to calcium-induced apoptosis and mammary gland involution. Persistent PMCA2 expression in certain breast cancers lowers calcium levels inside malignant cells, allowing them to avoid apoptosis. These tumors are also usually positive for the HER2 protein, tend to involve the lymph nodes, and are more common among young women, which could help explain their worse prognosis compared with postmenopausal women.
PMCAs were first discovered in the 1960s in the membranes of red blood cells. The presence of an ATPase was discovered in the membranes in 1961, and then in 1966 it was discovered that these ATPases pump Ca2+ out of the cytosol.
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