Plasma membrane Ca²⁺ ATPase

The plasma membrane Ca²⁺ ATPase (PMCA) is a transport protein in the plasma membrane of cells that serves to remove calcium (Ca²⁺) from the cell. It is vital for regulating the amount of Ca²⁺ within cells. [ cite journal | last =Jensen | first =TP | authorlink = | coauthors = Buckby LE; Empson RM| title = Expression of plasma membrane Ca²⁺ ATPase family members and associated synaptic proteins in acute and cultured organotypic hippocampal slices from rat. | journal =Brain Research. Developmental Brain Research. | volume =152 | issue = 2| pages =129–136 | publisher = | date =2004 | url = | doi = | pmid = 15351500 | accessdate =2007-01-13 ] In fact, the PMCA is involved in removing Ca²⁺ from all eukaryotic cells.cite journal | last =Strehler | first =EE | authorlink = | coauthors =Zacharias DA | title =Role of alternative splicing in generating isoform diversity among plasma membrane calcium pumps| journal =Physiological Reviews | volume = 81| issue = 1| pages =21–50 | publisher =American Physiological Society | date =2001 | url = http://physrev.physiology.org/cgi/content/full/81/1/21?ijkey=febc5196952b9604d7293e211729df5e16861525 | doi = | pmid =11152753 | accessdate =2007-01-30 ] There is a very large transmembrane electrochemical gradient of Ca²⁺ driving the entry of the ion into cells, yet it is very important for cells to maintain low concentrations of Ca²⁺ for proper cell signalling; thus it is necessary for the cell to employ ion pumps to remove the Ca²⁺.cite journal | last =Carafoli | first =E | authorlink = | coauthors = | title =Calcium pump of the plasma membrane | journal =Physiological Reviews | volume =71 | issue =1 | pages =129–153 | publisher = | date =1991 | url =http://physrev.physiology.org/cgi/reprint/71/1/129?ijkey=07dce5327b657cc65fb6152751c728391912b332&keytype2=tf_ipsecsha| doi = | pmid =1986387 | accessdate =2007-01-30 ] 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. cite journal | last =Talarico Jr | first =EF | authorlink = | coauthors =Kennedy BG; Marfurt CF; Loeffler KU; Mangini NJ | title = Expression and immunolocalization of plasma membrane calcium ATPase isoforms in human corneal epithelium.| journal =Molecular Vision | volume =11 | issue = | pages = 169–178| publisher = | date =2005 | url =http://www.molvis.org/molvis/v11/a19/ | doi = | id = | accessdate = ]

The PMCA belongs to a family of P-type primary ion transport ATPases that form an aspartyl phosphate intermediate.

The PMCA is expressed in a variety of tissues, including the brain. cite journal | last =Jensen | first =TP | authorlink = | coauthors =Filoteo A; Knopfel T; Empson RM. | title =Pre-synaptic plasma membrane Ca2+ ATPase isoform 2a regulates excitatory synaptic transmission in rat hippocampal CA3 | journal =Journal of Physiology | volume = | issue = | pages =Published online ahead of print | publisher = | date =2006 | url =http://jp.physoc.org/cgi/rapidpdf/jphysiol.2006.123901v1| doi = | id =17170045 | accessdate =2007-01-13 ]

Actions

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 K_m of 100 to 200 nM) but does not remove Ca²⁺ at a very fast rate.cite book | last =Siegel | first = GJ| authorlink = | coauthors =Agranoff BW, Albers RW, Fisher SK, Uhler MD, editors | title =Basic Neurochemistry: Molecular, Cellular, and Medical Aspects. 6th ed | publisher =Lippincott,Williams & Wilkins | date = 1999| location =Philadelphia | pages = | url =http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?cmd=Search&db=books&doptcmdl=GenBookHL&term=PMCA+AND+bnchm%5Bbook%5D+AND+160156%5Buid%5D&rid=bnchm.section.344#345 | doi = | id = Retrieved on January 13, 2007.] 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. [ cite journal | last = Burette | first =A | authorlink = | coauthors =Weinberg RJ | title =Perisynaptic organization of plasma membrane calcium pumps in cerebellar cortex | journal =Journal of Comparative Neurology | volume =500 | issue = 6| pages =1127–1135 | publisher = | date =2007 | url =| doi = | pmid =17183553 | accessdate =2007-01-30 ] 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.

The PMCA functions in a similar manner to other p-type ion pumps. ATP transfers a phosphate to the PMCA, which forms a phosphorylated intermediate.

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 ten fold.

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.

tructure

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.

Isoforms

There are four isoforms of PMCA, called PMCA 1 through 4.

* ATP2B1 - PMCA1
* ATP2B2 - PMCA2
* ATP2B3 - PMCA3
* ATP2B4 - PMCA4

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. cite journal | last =Fresu | first =L | authorlink = | coauthors = Dehpour A, Genazzani AA, Carafoli E, Guerini D | title =Plasma membrane calcium ATPase isoforms in astrocytes | journal =Glia | volume = 28| issue = 2| pages =150–155 | publisher = | date =1999 | url = | doi = | pmid =10533058 | accessdate =2007-01-30 ] Astrocytes help to maintain ionic balance in the extracellular space in the brain.

Knock-out of PMCA2 causes inner ear problems, including hearing loss and problems with balance.cite journal | last =Schuh | first =K | authorlink = | coauthors = Uldrijan S, Telkamp M, Rothlein N, Neyses L| title = The plasmamembrane calmodulin–dependent calcium pump : a major regulator of nitric oxide synthase I| journal =Journal of Cell Biology | volume =155 | issue =2 | pages =201–205 | publisher = The Rockefeller University Press| date =2001 | url =http://www.jcb.org/cgi/content/full/155/2/201 | doi =doi 10.1083 | pmid =11591728 | accessdate =2007-01-30 ]

PMCA4 exists in caveolae. Isoform PMCA4b interacts with nitric oxide synthase and reduces synthesis of nitric oxide by that enzyme.

PMCA isoform 4 has a molecular weight of 134,683, calculated from its sequence [ cite journal | last =Verma| first = AK| authorlink = | coauthors =Filoteo AG, Stanford DR, Wieben ED, Penniston JT, Strehler EE, Fischer R, Heim R, Vogel G, Mathews S, Strehler-Page M-A, James P, Vorherr T, Krebs J, Carafoli E | title =Complete Primary Structure of a Human Plasma Membrane Ca²⁺ Pump. | journal = The Journal of Biological Chemistry | volume =263| issue =28 | pages =14152–14159 | publisher = | date =1988 | url = http://www.jbc.org/| doi = | pmid = | accessdate =2008-09-01 ] . This is in good agreement with the results of SDS gel electrophoresis. [ cite journal | last =Graf| first = E| authorlink = | coauthors =Verma, AK, Gorski, JP, Lopaschuk, G, Niggli, V, Zurini, M, Carafoli, E, Penniston, JT | title =Molecular Properties of Calcium-Pumping ATPase from Human Erythrocytes. | journal = Biochemistry| volume =21| issue = | pages =4511-4516| publisher = | date =1982| url = http://acsinfo.acs.org/journals/bichaw/bichaw.html| doi = | pmid = | accessdate =2008-09-01 ] .

Pathology

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⁺.

History

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.

PMCA was first purified from red blood membranes in 1979 [cite journal | last =Niggli| first = V| authorlink = | coauthors =Penniston JT, Carafoli E | title =Purification of the (Ca²⁺-Mg²⁺ ATPase from Human Erythrocyte Membranes using a Calmodulin Affinity Column. | journal = The Journal of Biological Chemistry | volume =254| issue =20 | pages =9955–9958| publisher = | date =1979 | url = http://www.jbc.org/| doi = | pmid = | accessdate =2008-09-01.] [cite journal | last =Penniston| first = JT| authorlink = | coauthors = Filoteo AG, McDonough CS, Carafoli E | title =Purification Reconstitution and Regulation of Plasma Membrane Ca²⁺ Pumps. | journal = Methods in Enzymology| volume =157| issue = | pages =340–351| publisher = | date =1988| url = http://www.sciencedirect.com/science?_ob=ArticleURL&_udi=B7CV2-4B42SFD-39&_user=10&_coverDate=12%2F31%2F1988&_rdoc=31&_fmt=high&_orig=browse&_srch=doc-info(%23toc%2318066%231988%23998429999%23472660%23FLA%23display%23Volume)&_cdi=18066&_sort=d&_docanchor=&_ct=60&_acct=C000050221&_version=1&_urlVersion=0&_userid=10&md5=682083542638dd06234e5a1d7b4bacfc| doi = | pmid = | accessdate =2008-09-01.]

References

External links

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