Functional consequences of alterations to Glu309, Glu771, and Asp800 in the Ca(2+)-ATPase of sarcoplasmic reticulum

Molecular imaging of Na+,K(+)-ATPase in purified kidney membranes

Ion channels and pumps in cell membranes consist of multiple transmembrane segments that are thought to be critical for transport of ions. Channel structures constituted by these transmembrane segments are characteristic of ion channels, whereas such structures have not been identified in ion pumps until now. By applying atomic force microscopy on Na+,K(+)-ATPase molecules in canine kidney membranes under tapping mode, we identified a hollow in the protein with a characteristic internal diameter of 6-20 A and an external diameter of 20-55 A depending upon treatment conditions. This hollow may be interpreted as a channel-like conformation of Na+,K(+)-ATPase. In the regions where the proteins were absent, lipid head structures with 2 A width and 6 A length were imaged in an orthorhombic lattice.

Chimeric Ca(2+)-ATPase/Na+,K(+)-ATPase molecules. Their phosphoenzyme intermediates and sensitivity to Ca2+ and thapsigargin

Chimeric molecules consisting of parts from the sarcoplasmic reticulum Ca(2+)-ATPase and the Na+,K(+)-ATPase were expressed in COS-1 cells and analysed functionally. One chimera, in which most of the central cytoplasmic loop was derived from the Na+,K(+)-ATPase, while the transmembrane segments and the minor cytoplasmic loop came from the Ca(2+)-ATPase, was able to occlude Ca2+ and to be phosphorylated from ATP with normal apparent affinity for Ca2+ and ATP. This chimera also displayed normal sensitivity to thapsigargin, but was unable to undergo the transition from ADP-sensitive to ADP-insensitive phosphoenzyme and to transport Ca2+. The other chimera, which consisted of the NH2-terminal two-thirds of Na+,K(+)-ATPase and the COOH-terminal one-third of Ca(2+)-ATPase, was unable to phosphorylate from ATP, but phosphorylated from inorganic phosphate in a Ca(2+)-inhibitable and thapsigargin-insensitive reaction. These results can be explained in terms of a structural model in which the non-conserved residues in the central cytoplasmic domain of the Ca(2+)-ATPase are without major importance for the binding and occlusion of Ca2+, but are involved in the E1P-->E2P conformational changes of the phosphoenzyme, whereas residues in transmembrane segments on both sides of the central cytoplasmic domain are involved in formation of the Ca(2+)-binding sites. The data moreover show that thapsigargin sensitivity is dependent on residues in the NH2-terminal one-third of the Ca(2+)-ATPase molecule.

Site-directed mutagenesis of the Na,K-ATPase: consequences of substitutions of negatively-charged amino acids localized in the transmembrane domains

Site-directed mutagenesis was used to examine the importance of five carboxyl-containing amino acids localized in the putative membrane-spanning regions of the Na,K-ATPase (i.e., E327, E778, D803, D807, and D925 of the rat alpha 2 isoform). The substitutions were introduced into a cDNA encoding a ouabain-resistant isoform (i.e., rat alpha 2* which was mutated to encode a ouabain-resistant isoform), and the effect of these substitutions on Na,K-ATPase function was assessed by screening the altered enzymes for their ability to confer ouabain resistance when expressed in otherwise ouabain-sensitive cells. The expression of the alpha isoform containing certain substitutions at positions 327 and 925 was able to confer ouabain resistance to HeLa cells while the expression of rat alpha 2* containing substitutions at positions 778, 803, and 807 was not. In particular, amino acids in each of these positions were substituted with leucine to evaluate the importance of the carboxyl-containing side chain. The ability of rat alpha 2* containing E327L and D925L to confer ouabain resistance to HeLa cells indicates that neither the negative charge nor the oxygen-containing side chain is absolutely essential for overall function in this position. In contrast, the inability of rat alpha 2* carrying E778L, D803L, and D807L to confer ouabain resistance suggests that the naturally occurring amino acid may be more critical structurally and/or functionally for the Na,K-ATPase. Other more conservative substitutions introduced to further characterize the role of particular amino acid side chains include E327D, E327Q, D803N, D803E, and D925N.(ABSTRACT TRUNCATED AT 250 WORDS)

A Glu329-->Gln variant of the alpha-subunit of the rat kidney Na+,K(+)-ATPase can sustain active transport of Na+ and K+ and Na+,K(+)-activated ATP hydrolysis with normal turnover number

An allelic variant of the ouabain-insensitive rat kidney Na+,K(+)-ATPase alpha 1-isoform was identified by chance in a cDNA library. The variant differed from the wild-type rat kidney Na+,K(+)-ATPase by a single G-to-C base substitution in the cDNA, which on amino acid level gave rise to a glutamine in place of the glutamate residue Glu329 previously suggested as a likely donator of oxygen ligands for Na+ and K+ binding. The variant cDNA was transfected into COS-1 cells and the transfectants expanded with success into stable cell lines that were able to grow in the presence of a concentration of ouabain highly cytotoxic to the parental cells containing only the endogenous COS-1 cell Na+,K(+)-ATPase. Under these conditions, the viability of the cells depended on the cation transport mediated by the ouabain-insensitive Glu329-->Gln variant, whose cDNA was shown by polymerase chain reaction amplification to be stably integrated into the COS-1 cell genome. The maximum specific ATP hydrolysis activity of isolated plasma membranes of the Glu329-->Gln variant did not differ significantly from that of plasma membranes containing the wild type. A method was established for measurement of the phosphorylation capacity of the expressed Glu329-->Gln variant and wild-type enzyme, and it was thereby demonstrated that the variant had a turnover number similar if not identical to that of the wild-type.

Functional consequences of substitution of the seven-residue segment LysIleArgAspGlnMetAla240 located in the stalk helix S3 of the Ca(2+)-ATPase of sarcoplasmic reticulum

Site-directed mutagenesis was used to substitute the seven-residue segment LysIleArgAspGlnMetAla240 located at the NH2- terminal end of the "stalk" helix S3, near the beta-strand domain, in the sarcoplasmic reticulum Ca(2+)-ATPase of rabbit fast twitch muscle, with the corresponding Na+,K(+)-ATPase segment ArgIleAlaThrLeuAlaSer. This led to a new phenotypic variant of Ca(2+)-ATPase. The overall turnover rates for Ca2+ transport and ATP hydrolysis measured at 27 and 37 degrees C, respectively, were reduced to 30-40% of the wild-type rates. Analysis of the phosphoenzyme intermediates at 0 degrees C showed that the ADP-insensitive phosphoenzyme intermediate accumulated under conditions where the ADP-sensitive phosphoenzyme intermediate predominated in the wild-type Ca(2+)-ATPase. The rate of dephosphorylation of the ADP-insensitive phosphoenzyme intermediate formed through the forward reaction with ATP, or in the "backdoor" reaction with Pi, was reduced severalfold in the mutant relative to the dephosphorylation rate measured in the wild type, but there was no significant difference between the mutant and the wild type with respect to the apparent affinity for Pi measured under equilibrium conditions. The mutant was much less susceptible to inhibition by vanadate than the wild type, under equilibrium conditions as well as during turnover with ATP and Ca2+. These observations suggest that the transition state in the hydrolysis of the aspartyl phosphate bond in the ADP-insensitive phosphoenzyme intermediate was destabilized in the mutant.

Site-directed mutagenesis of a predicted cation binding site of Na, K-ATPase

Chemical modification and proteolytic digestion studies have identified a transmembrane glutamic acid residue (E953) of the alpha subunit of the pig kidney Na, K-ATPase as a possible cation binding site [Goldshleger et al. (1992) Proc. Natl. Acad. Sci. U.S.A. 89, 6911-6915]. In addition, an adjacent glutamate (E954) is conserved in all species and isoforms and may also be involved in cation binding. To further explore the role of these residues in ion transport, we have utilized a mutagenesis-expression strategy. This approach avoids the introduction of a large chemical moiety into the protein and allows specific amino acid substitutions to be introduced. Glutamic acid residues 955 and 956 of the rat alpha-1 subunit (corresponding to glutamates 953 and 954 of the pig kidney Na, K-ATPase) were replaced separately and together using site-directed mutagenesis of the rat alpha-1 cDNA. The mutant cDNAs were expressed in ouabain-sensitive HeLa cells. This system makes it possible to rapidly identify amino acid substitutions which significantly impair enzyme function, as substitutions which do not affect enzyme activity will yield colonies in the presence of ouabain, while substitutions which severely impair function will prevent or limit growth of the ouabain-sensitive HeLa cells. The amino acid replacements (E955Q, E956Q, E955Q-E956Q, E955D-E956D) all resulted in the growth of ouabain-sensitive cells, demonstrating that the modified Na, K-ATPase in each case was functional. To further study the altered enzymes, ouabain-resistant colonies were isolated and expanded into stable cell lines.(ABSTRACT TRUNCATED AT 250 WORDS)