Mammalian Calcium Pumps in Health and disease

Ca2+ 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.