1,721,173 research outputs found
Calcium pumps: structural basis for and mechanism of calcium transmembrane transport.
No full textEukaryotic cells remove calcium from the cytosol using P-type pumps in the plasma membrane and in the sarco(endo)plasmic reticulum. These pumps share membrane topography and general mechanism of action, but differ in regulatory properties. Recent advances in the field include the three-dimensional structure of the sarco(endo)plasmic reticulum and further understanding of the transcriptional regulation of the plasma membrane P-type pump by calcium
Calcium Pumps in Health and Disease
No full textCa2+-ATPases (pumps) are key actors in the regulation of Ca2+ in eukaryotic cells and are thus essential to the correct functioning of the cell machinery. They have high affinity for Ca2+ and can efficiently regulate it down to very low concentration levels. Two of the pumps have been known for decades (the SERCA and PMCA pumps); one (the SPCA pump) has only become known recently. Each pump is the product of a multigene family, the number of isoforms being further increased by alternative splicing of the primary transcripts. The three pumps share the basic features of the catalytic mechanism but differ in a number of properties related to tissue distribution, regulation, and role in the cellular homeostasis of Ca2+. The molecular understanding of the function of the pumps has received great impetus from the solution of the three-dimensional structure of one of them, the SERCA pump. These spectacular advances in the structure and molecular mechanism of the pumps have been accompanied by the emergence and rapid expansion of the topic of pump malfunction, which has paralleled the rapid expansion of knowledge in the topic of Ca2+-signaling dysfunction. Most of the pump defects described so far are genetic: when they are very severe, they produce gross and global disturbances of Ca2+ homeostasis that are incompatible with cell life. However, pump defects may also be of a type that produce subtler, often tissue-specific disturbances that affect individual components of the Ca2+-controlling and/or processing machinery. They do not bring cells to immediate death but seriously compromise their normal functioning
Mammalian Calcium Pumps in Health and disease
No full textCa2+ transporting ATPases (Ca2+ pumps) have been described in animal and in plant cells and in cells of lower eukaryotes. This contribution will focus on those of animal cells and on the disease processes linked to their dysfunction. The three animal Ca2+ pumps belong to the large superfamily of P-type ATPases, which have been so defined [1] because their reaction cycle is characterized by the formation of an acid-stable phosphorylated Asp residue (the P intermediate) in a highly conserved sequence (SDKTGT[L/IV/M][T/I/S]). The family now contains hundreds of members and eight sub-families [2]. The sub-families have been identified based essentially on transported substrate specificity, the evolutionary appearance of which having been accompanied by abrupt changes in sequence. The changes, however, do not involve eight conserved structurally and mechanistically important regions which define the core of the superfamily. Five branches have been identified in the phylogenetic tree of the superfamily: two animal Ca2+ pumps belong to subgroup II A (the SERCA and SPCA pumps), one to subgroup II B (the PMCA pump). All P-type ATPases, including the three that transport Ca2+ in animal cells, are multidomain proteins that share the essential properties of the reaction mechanism, have molecular masses varying between 70 and 150 kDa, and share the presence of 10 hydrophobic membrane spanning domains (TM) (some, however, only have six or eight). The number of TMs being even, the N- and C-termini of all P-type pumps are on the same membrane side, i.e., the cytosol: one exception is a splice variant of the SERCA pump that has 11 TM, see below). The P-type ATPases also share the sensitivity to the transition state analogue orthovanadate and, with some specific differences (see below), to La3+. Other inhibitors, only affect selected members of the superfamily. The 3D structures of four P-type ATPases have now become available following the landmark solution of that of the SERCA pump 12 years ago [3]: molecular modeling on templates of the SERCA pump structure has indicated that all P-type ATPases share the general principles of 3D structure. The reaction cycle of P-type ATPases originally envisaged only the E1 and E2 steps, characterized by distinct conformations and affinities for ATP and the transported ion: in Ca2+ pumps, for instance, in the E1 state the pump engages Ca2+ with high affinity at one side of the membrane, and in the E2 state its lowered affinity for Ca2+ releases it to the opposite membrane side [4]. Later on, additional intermediate states were added that made the reaction cycle much more complex, but the basic E1/E2 nomenclature has been retained. Importantly, each step of the reaction cycle is reversible, so that ATP can be produced by reversing the direction of the ion transport process: reversal of the SERCA pump, with production of ATP, had in fact already been demonstrated in one of the first experiments on the transport of Ca2+ by vesicular preparations of sarcoplasmic reticulum [5]. A simplified version of the cycle, but adapted to Ca2+ pumps, is shown in Figure 1. Several Ca2+ pump isoforms have been described in animal cells, differing essentially in tissue distribution, regulatory properties, and some mechanistic peculiarities. The isoform diversity reflects the existence of separate basic gene products, but also the occurrence of complex patterns of alternative splicing that increase very significantly the number of variants of each of the three pumps. The analysis of the differential properties of the Ca2+ pump isoforms is now a vigorously investigated topic that has important linkages to the general process of cellular Ca2+ homeostasis: which in animal cells is regulated by a number of non-membrane Ca2+ binding proteins and of membrane intrinsic Ca2+ channels and transporters. The transporters interact with Ca2+ with high or low affinity, and thus function either as fine tuners of cytosolic Ca2+ or come into play whenever the concentration of Ca2+ increases to levels adequate for their low affinity. The Na/Ca-exchanger of the plasma membrane and the mitochondrial Ca2+ uptake and release systems are the low affinity regulators of cytosolic Ca2+. The three pumps, by contrast, control Ca2+ efficiently even in the low concentrations of the cytosol at rest. Their activity is fundamental to the correct functioning of the machinery of animal cells: dysfunctions, genetic or otherwise, of their operation, may not necessarily induce cell death, but invariably generate disease phenotypes: they now define a topic that has recently grown very impressively
Recombinant expression of the plasma memembrane Na+/Ca2+ exchanger affects local and global Ca2+ homeostasis in Chinese hamster ovary cells
No full textThe cardiac type Na+/Ca2+ exchanger (NCX1) has been transiently expressed in Chinese hamster ovary cells, which do not contain an endogenous exchanger, together with aequorin chimeras that are targeted to different intracellular compartments to investigate intracellular Ca2+ homeostasis. The expression of NCX decreased the endoplasmic reticulum Ca2+ concentration, [Ca2+](er), in resting cells, showing that the exchanger was operative under these conditions. It induced a greater reduction in the height of the mitochondrial and cytosolic Ca2+ transients in agonist-stimulated cells than would have been expected from the [Ca2+](er) decrease. It also had a major effect on the sub-plasma membrane Ca2+ concentration, [Ca2+](pm): after a transient [Ca2+](pm) rise induced by the activation of capacitative Ca2+ influx, [Ca2+](pm) settled to a value about 3-fold higher than in controls. The sustained [Ca2+](pm) increase after the transient was due to the operation of the exchanger, either directly by operating in the Ca2+ entry mode, or indirectly by removing the Ca2+ inhibition on the capacitative Ca2+ influx channels
Transcriptional control of the Na+/ Ca2+ exchanger
No full textThe expression of NCX genes is controlled by diverse signal transduction pathways. The transcription of the NCX2 gene during the maturation of cerebellar granule neurons is rapidly down-regulated by calcium in a precess that is mediated by calcineurin, whereas that of NCX1 and NCX2 is not affected. The structure of NCX1 gene is not knowns. An unusually large exon, which has been found aòlso in NCX3 gene encodes about two thirds of the full-length NCx1 protein. This large exon (exon 2) encodes a functional exchanger truncated after Val 600
Nicotinic acid adenine dinucleotide phosphate-induced Ca2+-release: interactions among distinct Ca2+ mobilizing mechanisms in starfish oocytes
No full textRecombinant expression of the plasma membrane Na+/Ca2+ exchanger affects local and global Ca2+ homeostasis in CHO cells
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