Intracellular signaling through long-range linked functions in the Ca2+ transport ATPase

Expression of a prokaryotic P-type ATPase in E. coli Plasma Membranes and Purification by Ni2+-affinity chromatography

In order to characterize the P-type ATPase from Synechocystis 6803 [Geisler (1993) et al. J. Mol. Biol. 234, 1284] and to facilitate its purification, we expressed an N-terminal 6xHis-tagged version of the ATPase in an ATPase deficient E. coli strain. The expressed ATPase was immunodetected as a dominant band of about 97 kDa localized to the E. coli plasma membranes representing about 20-25% of the membrane protein. The purification of the Synecho-cystis 6xHis-ATPase by single-step Ni-affinity chromatography under native and denaturating conditions is described. ATPase activity and the formation of phosphointermediates verify the full function of the enzyme: the ATPase is inhibited by vanadate (IC(50)= 119 &mgr;M) and the formation of phosphorylated enzyme intermediates shown by acidic PAGE depends on calcium, indicating that the Synechocystis P-ATPase functions as a calcium pump.

Expression and characterization of a Synechocystis PCC 6803 P-type ATPase in E. coli plasma membranes

In a previous paper, we published the sequence of a P-type ATPase gene from Synechocystis 6803 [Geisler et al. (1993) J. Mol. Biol. 234, 1284] which showed significant homologies to eukaryotic calcium ATPases. To investigate the specificity and activities of this plasma membrane-bound enzyme, we expressed the slightly modified gene in an ATPase deficient E. coli strain. The expressed ATPase showed an apparent molecular mass of about 97kDa and is localized in the E. coli plasma membranes. The introduced 6xHis tag at the N-terminus allowed the purification of the Synechocystis 6xHis-ATPase by single-step affinity chromatography using a Ni2+-nitrilotriacetic acid resin. The ATPase activity of the enzyme is inhibited by vanadate (IC50 = 119 microM), N-ethylmaleimide, N,N-dicyclohexylcarbodiimide, and inhibitors of eukaryotic sarco(endo)plasmic reticulum Ca2+-ATPases; however, it is stimulated by thapsigargin. Formation of phosphorylated enzyme intermediates depends on calcium ions indicating that the Synechocystis P-ATPase acts as a calcium pump equivalent to eukaryotic sarco(endo)plasmic reticulum Ca2+-ATPases.

PMR1, a Ca2+-ATPase in yeast Golgi, has properties distinct from sarco/endoplasmic reticulum and plasma membrane calcium pumps

PMR1, a P-type ATPase cloned from the yeast Saccharomyces cerevisiae, was previously localized to the Golgi, and shown to be required for normal secretory processes (Antebi, A., and Fink, G.R. (1992) Mol. Biol. Cell 3, 633-654). We provide biochemical evidence that PMR1 is a Ca2+-transporting ATPase in the Golgi, a hitherto unusual location for a Ca2+ pump. As a starting point for structure-function analysis using a mutagenic approach, we used the strong and inducible heat shock promoter to direct high level expression of PMR1 from a multicopy plasmid. Yeast lysates were separated on sucrose density gradients, and fractions assayed for organellar markers. PMR1 is found in fractions containing the Golgi marker guanosine diphosphatase, and is associated with an ATP-dependent, protonophore-insensitive 45Ca2+ uptake activity. This activity is virtually abolished in the absence of the expression plasmid. Furthermore, replacement of the active site aspartate within the phosphorylation domain had the expected effect of abolishing Ca2+ transport activity entirely. Interestingly, the mutant enzymes (Asp-371 --> Glu and Asp-371 --> Asn) demonstrated proper targeting to the Golgi, unlike analogous mutations in the related yeast H+-ATPase. Detailed characterization of calcium transport by PMR1 showed that sensitivity to inhibitors (vanadate, thapsigargin, and cyclopiazonic acid) and affinity for substrates (MgATP and Ca2+) were different from the previously characterized sarco/endoplasmic reticulum and plasma membrane Ca2+-ATPases. PMR1 therefore represents a new and distinct P-type Ca2+-ATPase. Because close homologs of PMR1 have been cloned from rat and other organisms, we suggest that Ca2+-ATPases in the Golgi will form a discrete subgroup that are important for functioning of the secretory pathway.

Catalytic and structural modifications of sarcoplasmic reticulum and plasma membrane (Ca(2+) + Mg2+) ATPases induced by organic solutes that accumulate in living systems

Organic solutes such as urea, methylamines, polyols and amino acid can accumulate in the cytoplasm of cells to compensate for hyperosmotic conditions in the external medium. Whereas urea is considered to be typical of solutes that destabilize structure and function of proteins, methylamines, polyols and some amino acids appear to have the opposite effect, and can also compensate for the perturbing effects of urea. These effects have been extensively analyzed for a variety of proteins in terms of global changes in enzyme structure and acceleration or inhibition of overall reaction rates. Here the influence of these solutes on sarcoplasmic reticulum and plasma membrane (Ca(2+) + Mg2+) ATPases is reviewed. The focus is on the changes induced by "perturbing" and "stabilizing" solutes at specific steps of the catalytic cycles of these enzymes, which can run forward (leading to ATP hydrolysis) and backward (leading to ATP synthesis). Structural changes promoted by osmolytes are correlated with functional changes, especially those that are related to energy coupling.

Site-directed mutagenesis of the yeast PMA1 H(+)-ATPase. Structural and functional role of cysteine residues

The yeast plasma-membrane H(+)-ATPase contains nine cysteines, three in presumed transmembrane segments (Cys-148, Cys-312, and Cys-867) and the rest in hydrophilic regions thought to be exposed at the cytoplasmic surface (Cys-221, Cys-376, Cys-409, Cys-472, Cys-532, and Cys-569). To gather new functional and structural information, we have studied the yeast ATPase by cysteine mutagenesis. It proved possible to replace seven of the nine cysteines by alanine, one at a time, without any significant decrease in ATP hydrolysis or ATP-dependent proton pumping. In the remaining two cases (Cys-409 and Cys-472), there were small but reproducible effects; the results clearly indicated, however, that no single Cys is required for activity and that, if a disulfide bridge is formed in the yeast ATPase, it does not play an obligatory structural or functional role. Next, multiple mutants were constructed to ask how many Cys residues could be replaced simultaneously while leaving a fully functional enzyme. After substitution of all "membrane" Cys (Cys-148, Cys-312, and Cys-867) together with two non-conserved Cys located in hydrophilic regions (Cys-221 and Cys-569), there were no significant abnormalities in expression (87%) or activity (89% ATP hydrolysis/93% H+ pumping) of the mutant protein. Replacement of two additional cysteines (Cys-376 near the phosphorylation site and Cys-532, in or near the ATP-binding site) caused a drop in expression (to 54%), although the corrected hydrolytic and H+ pumping activities were still normal. When Cys-472 was also mutated, the corrected activity fell to 44% hydrolysis/47% pumping; finally, substitution of Cys-409 to give a "cysteine-free" ATPase led to a very poorly expressed and poorly active enzyme. Brief exposure of the "one-cysteine" and "two-cysteine" ATPases to trypsin revealed a normal pattern of degradation, but there was a slight impairment in the ability of vanadate to protect against proteolysis. Thus, although single Cys replacements are tolerated well by the yeast ATPase, multiple replacements are progressively more harmful, suggesting that they cause small but additive perturbations of protein folding.