Ca2+ acts as a universal second messenger in intracellular signaling in eukaryotes. The maintenance and regulation of the free intracellular Ca2+ concentration requires the presence of specialized Ca2+ transporters in the plasma membrane and in intracellular organellar membranes. Na+/Ca2+ exchangers and ATP-driven Ca2+ pumps are the two major systems responsible for uphill Ca2+ transport against its large concentration gradient. Na+/Ca2+ exchangers are high capacity transporters involved in the handling of large and dynamic Ca2+ fluxes across the plasma membrane. Cardiac-type exchangers are ubiquitous but are most abundant in excitable cells such as in heart and nervous tissue. They use the energy stored in the electrochemical Na+ gradient to exchange one Ca2+ for three Na+ per reaction cycle. In the forward mode they expel Ca2+ from the cell in exchange for Na+. However, depending on the prevailing Na+ and Ca2+ ion distribution and on the membrane potential they can also operate in the reverse mode. The photoreceptor exchanger is highly specific for the outer membrane segment of vertebrate retinal rods and normally operates with a 4Na+/1Ca2+, 1K+ stoichiometry. The cardiac-and photoreceptor-type exchangers are single polypeptides with native molecular masses of about 120 and 220 kDa, and cDNA-derived calculated masses of about 105 and 130 kDa, respectively. The differences are mainly due to glycosylation on their N-terminal extracellular segments. Both exchanger types contain a leader peptide which is cleaved during maturation, and they show extensive structural similarities in their predicted transmembrane topology: two clusters of five and six transmembrane segments close to the N-and C-termini, respectively, are separated by a large cytosolic hydrophilic domain. 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 for Na+/Ca2+ exchange, PMCAs are the sole mechanism of specific Ca2+ export. They play an essential role in the long-term maintenance and the resetting of resting free Ca2+ levels. SERCAs are responsible for the rapid Ca2+ reuptake into the endoplasmic/sarcoplasmic reticulum and its specialized compartments after Ca2+ signaling. Ca2+ pumps belong to the class of P-type pumps which are characterized by the formation of a phosphorylated intermediate in which the energy derived from ATP is transiently stored as conformational constraint. Ca2+ pumps shuttle between two major conformational states E1 and E2. In the E1 state they bind Ca2+ on the cytosolic (cis) side with very high affinity (KCa≈0.5 μM). Upon Ca2+ binding their ATPase activity is stimulated, leading to the formation of the phosphorylated intermediate and followed by occlusion of the transported Ca2+, transition to the low energy, low Ca2+ affinity E2 state and release of Ca2+ on the trans side. Two Ca2+ ions are transported per ATP split in SERCAs but only one in the PMCAs. SERCAs and PMCAs are single polypeptides with molecular masses of about 100 and 130 kDa. Despite limited primary sequence identity their predicted transmembrane topologies are very similar and consits of two clusters of four and six transmembrane segments separated by a large cytosolic domain containing the catalytic and ATP binding sites. Polar amino acid side chains within at least four transmembrane segments participate in forming the high affinity Ca2+ binding/transport sites in SERCAs, and probably in PMCAs, too. Long-range conformational interactions must occur in the pumps to enable changes in the catalytic site region to affect Ca2+ binding and translocation in the transmembrane region. PMCAs, but not SERCAs, are directly stimulated by Ca2+-calmodulin; indeed, most of the additional mass of PMCAs with respect to SERCAs is due to a C-terminal regulatory region containing the autoinhibitory, calmodulin-binding domain. Multiple isoforms of SERCAs and PMCAs are generated from multigene families and from alternatively spliced mRNAs. The tissue and developmental stage-specific expression of several SERCA and PMCA isoforms suggests that specific functional and/or regulatory properties of these pumps are adapted to the physiological needs of a given cell type.
ASJC Scopus subject areas
- Biochemistry, Genetics and Molecular Biology(all)