Membrane Ca2+-ATPase

The plasma membrane Ca2+-ATPase pump effects outward transport of Ca2+ against a large electrochemical gradient for Ca2+. The mechanism of the pump involves its phosphorylation by ATP and the formation of a high-energy intermediate. This basic mechanism is similar for both the plasma membrane and ER pumps however, the structures of these distinct gene products are substantially different. As discussed below, the ER pump, sometimes called a sarcoendoplasmic reticulum Ca2+-ATPase (SERCA) pump, is inhibited potently by certain natural and synthetic toxins that do not affect the plasma membrane pump. The plasma membrane pump, but not the SERCA pump, is controlled in part by Ca2+ calmodulin, allowing for rapid activation when cytoplasmic Ca2+ rises. 

As both fast and slow adaptation mechanisms are regulated by Ca2+, the stereocilia mechanisms that control the free concentration of this ion also play central roles in transduction. Entering Ca2+ is thought to be buffered very rapidly by the mobile buffers parvalbumin 3, calbindin, and calretinin [22,23]. Even before bound Ca2+ can diffuse out of stereocilia, it is pumped back out into the endo-lymph by isoform 2a of the plasma-membrane Ca2+-ATPase (PMCA2) [24,25] (see also Ca2+ transport in Ch. 5). 

B. L. and Noben-Trauth, K. Mutations in a plasma membrane Ca2+-ATPase gene cause deafness in deafwaddler mice. Nat. Genet. 19 390-394,1998. 

Kozel, P. J. et al. Balance and hearing deficits in mice with a null mutation in the gene encoding plasma membrane Ca2+-ATPase isoform 2. /. Biol. Chem. 273 18693-18696,1998. 

Mammalian cells maintain a lower concentration of Na+ (around 12 mM) and a higher concentration of K+ (around 140 mM) than in the surrounding extracellular medium (respectively, 145 and 4 mM). The system (Na+-K+)-ATPase, which maintains high intracellular K+ and low intracellular Na+, is localized in the plasma membrane, and belongs to the family of P-type ATPases. Other members of the family in eukaryotes are the sarcoplasmic reticulum and plasma membrane Ca2+ ATPases and, in plants, the H+-ATPases. The overall reaction catalysed is ... 

Figure 11.8). In addition, the activated enzyme phosphorylates itself, and thus remains partly active even after the Ca2+ concentration falls and calmodulin is released from the enzyme. In contrast to the CaM kinases, another important target of Ca2+-cahnodulin is the plasma membrane Ca2+-ATPase pump, whose activation drives down the Ca2+ concentration within the cell, helping to terminate the signal. 

A large and diverse group of proteins, including enzymes, cytoskeleton, contractile proteins, and receptors, have been shown to be modified by calpains. Thus, a number of enzymes such as tyrosine hydrolase, tryptophan hydrolase, transglutaminase, protein kinase C, and membrane Ca2+-ATPase are activated by calpain proteolysis [38]. Several receptor proteins, in particular receptors for steroid hormones, growth factors, and adrenaline, are modulated by calpains, which participate also in platelet activation, cell fusion, and mitosis [39], Although the physiological roles of calpains continue to be un-... 

Chicka, M.C., Strehler, E.E., 2003, Alternative splicing of the first intracellular loop of plasma membrane Ca2+ ATPase isoform 2 alters its membrane targeting. J Biol Chem 16, 18464—18470. 

Hill, J.K., Williams, D.E., LeMasurier, M., Dumont, R.A., Strehler, E.E., Gillespie, P.G., 2006, Splice-site A choice targets Plasma-Membrane Ca2+-ATPase isoform 2 to hair bundles. J Neurosci 26, 6172-6180. 

Jensen, T.P., Filoteo, A., Knopfel, T., Empson, R.M., 2007, Pre-synaptic plasma membrane Ca2+ ATPase isoform 2a regulates excitatory synaptic transmission in rat hippocampal CA3. J Physiol (Lond) 15,... 

Kozel, P.J., Friedman, R.A., Erway, L.C., Yamoah, E.N., Liu, L.H., Riddle, T., Duffy, J.J., Doetschman, T., Miller, M.L., Cardell, E.L., Shull, G.E., 1998, Balance and hearing deficits in mice with a null mutation in the gene encoding plasma membrane Ca2+-ATPase isoform 2. J Biol Chem 273,... 

Nabekura, T., Tomohiro, M., Ito, Y., Kitagawa, S., 2004, Changes in plasma membrane Ca2+ -ATPase expression and ATP content in lenses of hereditary cataract UPL rats. Toxicology 197, 177-183. 

Okunade, G.W., Miller, M.L., Pyne, G.J., Sutliff, R.L., O Connor, K.T., Neumann, J.C., Andringa, A., Miller, D.A., Prasad, V., Doetschman, T., Paul, R.J., Shull, G.E., 2004, Targeted ablation of plasma membrane Ca2+-ATPase (PMCA) 1 and 4 indicates a major housekeeping function for PMCA1 and a critical role in hyperactivated sperm motility and male fertility for PMCA4. J Biol Chem 279, 33742-33750. 

Rosado, J.A., Saavedra, F.R., Redondo, P.C., Hernandez-Cruz, J.M., Salido, G.M., Pariente, J.A., 2004, Reduced plasma membrane Ca2+-ATPase function in platelets from patients with non-insulin-dependent diabetes mellitus. Haematologica 89, 1142-1144. 

Saito, K., Uzawa, K., Endo, Y., Kato, Y., Nakashima, D., Ogawara, K., Shiba, M., Bukawa, H., Yokoe, H., Tanzawa, H., 2006, Plasma membrane Ca2+ ATPase isoform 1 down-regulated in human oral cancer. Oncol Rep 15, 49-55. 

Silverstein, R.S., Tempel, B.L., 2006, Atp2b2, encoding plasma membrane Ca2+-ATPase type 2, (PMCA2) exhibits tissue-specific first exon usage in hair cells, neurons, and mammary glands of mice. Neuroscience 141, 245-257. 

Wan, T.C., Zabe, M., Dean, W.L., 2003, Plasma membrane Ca2+-ATPase isoform 4b is phosphorylated on tyrosine 1176 in activated human platelets. Thrombosis and haemostasis 89, 122-131. 

Delgado-Coello, B., Santiago-Garcia, J., Zarain-Herzberg, A. and Mas-Oliva, J., 2003, Plasma membrane Ca2+-ATPase mRNA expression in murine hepatocarcinoma and regenerating liver cells. Mol Cell Biochem 247, 177-84. 

Stains JP, Weber JA, Gay CV. 2002. Expression of Na+/Ca2+ exchanger isoforms (NCX1 and NCX3) and plasma membrane Ca2+-ATPase during osteoblast differentiation. J Cell Biochem 84 625-35. 

Keeton, T.P., Burk, S.E., Shull, G.E. (1993). Alternative splicing of exons encoding the calmodulin-binding domains and C termini of plasma membrane Ca2+-ATPase isoforms 1,2,3, and 4. J. Biol. Chem. 268,2740-2748. 

Ca2+ pumps are divided into two distinct families of plasma membrane and sarco/endoplasmic reticulum membrane Ca2+ ATPases (PMCAs and SERCAs). In tissues with a low Na+/Ca2+ exchanger activity and under conditions unfavorable... 

Complexes of PolyP and PHB were found in the membranes of the endoplasmic reticulum and mitochondria of animal cells (Reusch, 1989), which suggests their participation in the processes of transmembrane transfer. The most intriguing report was that the Ca2+-ATPase purified from human erythrocytes contains PolyP and PHB and that the plasma membrane Ca2+-ATPase may function as a polyphosphate kinase it exhibits ATP-PolyP transferase and PolyP-ADP transferase activities (Reusch et al, 1997). These findings suggest a novel supramolecular structure for the functional Ca2+-ATPase and a new mechanism of uphill Ca2+ extrusion coupled with ATP hydrolysis (Reusch et al., 1997). 

In addition to mobilizing internal Ca2+, glucagon promotes the net entry of extracellular Ca2+ into hepatocytes [144,145]. The effect is apparently due to increased influx of Ca2+ through a plasma membrane channel(s) [144,145], but there is also inhibition of the plasma membrane Ca2+ ATPase-pump [150-153], The mechanism by which glucagon stimulates Ca2+ entry is unknown, but it almost certainly involves cAMP since the effect can be mimicked by forskolin, dibutyryl cAMP and /3-adrenergic agonists [144,145]. It may involve cAMP-dependent phosphorylation of a Ca2+ channel analogous to the situation in cardiac and skeletal muscle [154-156], but this is strictly speculative. 

Increased cAMP in smooth musle cells causes muscle relaxation through activation of cAMP-dependent PK (PKA). PKA phosphorylates MLGK and phospho-MLCK (P-MLGK) is poorly activated by Ca2+-CaM. PKA also phosphorylates an ER protein called phospholamban, the P-phospholamban entity increasing the activity of the ER membrane Ca2+-ATPase which lowers cytosolic Ca2+ concentration and thus prevents smooth muscle contraction. 

Ca2+-ATPases exist in the plasma membranes of most cells and in the sarcoplasmic reticulum of myocytes, where they pump Ca2+out of the cytosol and into the lumen, respectively, while simultaneously counterporting H+ions. Ca2+-ATPase requires Mg2+on the side from which Ca2+is pumped. It is generally established that the Ca2+/ATP stoichiometries for the plasma membrane and sarcoplasmic reticulum are 1 and 2, respectively. Using a nonequilibrium thermodynamics model, the extent of slippage in the plasma membrane Ca2+-ATPase can be estimated from steady-state H+flow measurements. 

Using Eq. (11.141), the extent of slippage Lp JLp can be estimated in the plasma membrane Ca2+-ATPase. This is done close to the static head without the ionophore (-), and close to the level flow with the ionophore (+). Using the equation for./p. the control ratio of the ATPase is obtained from... 

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