Calcium transport by sarcoplasmic reticulum Ca(2+)-ATPase. Role of the A domain and its C-terminal link with the transmembrane region

Regulation of Na,K-ATPase by PLMS, the phospholemman-like protein from shark: molecular cloning, sequence, expression, cellular distribution, and functional effects of PLMS

In Na,K-ATPase membrane preparations from shark rectal glands, we have previously identified an FXYD domain-containing protein, phospholemman-like protein from shark, PLMS. This protein was shown to associate and modulate shark Na,K-ATPase activity in vitro. Here we describe the complete coding sequence, expression, and cellular localization of PLMS in the rectal gland of the shark Squalus acanthias. The mature protein contained 74 amino acids, including the N-terminal FXYD motif and a C-terminal protein kinase multisite phosphorylation motif. The sequence is preceded by a 20 amino acid candidate cleavable signal sequence. Immunogold labeling of the Na,K-ATPase alpha-subunit and PLMS showed the presence of alpha and PLMS in the basolateral membranes of the rectal gland cells and suggested their partial colocalization. Furthermore, through controlled proteolysis, the C terminus of PLMS containing the protein kinase phosphorylation domain can be specifically cleaved. Removal of this domain resulted in stimulation of maximal Na,K-ATPase activity, as well as several partial reactions. Both the E1 approximately P --> E2-P reaction, which is partially rate-limiting in shark, and the K+ deocclusion reaction, E2(K) --> E1, are accelerated. The latter may explain the finding that the apparent Na+ affinity was increased by the specific C-terminal PLMS truncation. Thus, these data are consistent with a model where interaction of the phosphorylation domain of PLMS with the Na,K-ATPase alpha-subunit is important for the modulation of shark Na,K-ATPase activity.

Inhibition of human SERCA3 by PL/IM430. Molecular analysis of the interaction

The monoclonal antibody PL/IM430 has previously been reported to uncouple Ca(2+) transport from ATP hydrolysis in platelet membranes (Hack, N., Wilkinson, J. M., and Crawford, N. (1988) Biochem. J. 250, 355-361). More recently, we have demonstrated that this antibody is specific for human SERCA3 (Poch, E., Leach, S., Snape, S., Cacic, T., MacLennan, D. H., and Lytton, J. (1998) Am. J. Physiol. 275, C1449-C1458). In this paper, we have extended the analysis of the PL/IM430-SERCA3 interaction. Using HEK293 cells to express human SERCA3a, we were able to measure both ATP-mediated, oxalate-dependent (45)Ca(2+) uptake and Ca(2+)-dependent ATP hydrolysis activities due exclusively to SERCA3. Treatment with PL/IM430 inhibited both activities almost identically, with a maximal inhibition of 81 and 73% and a half-maximal concentration of 8.3 and 5.9 microg/ml, for Ca(2+) uptake and ATP hydrolysis, respectively. We conclude that PL/IM430 does inhibit SERCA3 activity but does not uncouple Ca(2+) transport from ATP hydrolysis. Using a combination of partial proteolysis, GST fusion protein expression, and mutation of residues that differ between rat and human SERCA3, we have identified human SERCA3 amino acids Pro(8) and Glu(192) as essential to forming the PL/IM430 epitope. PL/IM430 thus recognizes a linearly noncontiguous set of amino acids within the actuator domain of human SERCA3. We propose that PL/IM430 inhibits SERCA3 activity by sterically preventing movement of the actuator domain into a catalytically critical position in the E2 conformation of the enzyme.

Importance of conserved Thr214 in domain A of the Na+,K+ -ATPase for stabilization of the phosphoryl transition state complex in E2P dephosphorylation

Thr(214) of the highly conserved (214)TGES sequence in domain A of the Na(+),K(+)-ATPase was replaced with alanine, and the mutant was compared functionally with the previously characterized domain A mutant Gly(263) --> Ala. Thr(214) --> Ala displayed a conspicuous 150-fold reduction of the apparent vanadate affinity for inhibition of ATPase activity, which could not simply be explained by the observed shifts of the conformational equilibria in favor of E(1) and E(1)P. The intrinsic vanadate affinity of the E(2) form and the effect on the apparent vanadate affinity of displacement of the E(1)-E(2) equilibrium were determined in a phosphorylation assay that allows the enzyme-vanadate complex to be formed under equilibrium conditions. When the E(2) form prevailed, Thr(214) --> Ala retained a reduced vanadate affinity relative to wild type, whereas the affinity of Gly(263) --> Ala became wild type-like. Thus, mutation of Thr(214) affected the intrinsic affinity of E(2) for vanadate. Furthermore, Thr(214) --> Ala showed at least a 5-fold reduced E(2)P dephosphorylation rate relative to wild type in the presence of saturating concentrations of K(+) and Mg(2+). Because vanadate is a phosphoryl transition state analog, it is proposed that defective binding of the phosphoryl transition state complex (transition state destabilization) causes the inability to catalyze E(2)P dephosphorylation properly. By contrast, the phosphorylation site in the E(1) form was unaffected in Thr(214) --> Ala. Replacement of the glutamate, Glu(216), of (214)TGES with alanine was incompatible with cell viability, indicating a very low transport activity or expression level. Our results support the hypothesis that domain A is isolated in the E(1) form, but contributes to make up the catalytic site in the E(2) and E(2)P conformations.

Roles of Leu249, Lys252, and Leu253 in membrane segment M3 of sarcoplasmic reticulum Ca2+-ATPase in control of Ca2+ migration and long-range intramolecular communication

Point mutants with alterations to Leu249, Lys252, Leu253, Asp254, and Glu255 in membrane segment M3, and Pro824, Lys825, and Glu826 in loop L6-7, of the sarcoplasmic reticulum Ca2+-ATPase were analyzed functionally by steady-state and transient kinetic methods. In mutants Leu249Ala, Lys252Glu, and Leu253Ala, the rate of Ca2+ dissociation from the cytoplasmically facing high-affinity Ca2+ sites was increased 4- to 7-fold relative to wild type, and in Leu249Ala and Lys252Glu the rate of Ca2+ binding was increased as well. Substitution of Lys252 with arginine, alanine, glutamine, or methionine affected Ca2+ interaction much less, indicating that the negative charge of the glutamate is particularly disturbing. These findings may be understood on the basis of the hypothesis that a water-accessible channel leading between membrane segments M1 and M3 in the thapsigargin-bound Ca2+-free structure [Toyoshima, C., and Nomura, H. (2002) Nature 418, 605-611] is closely related to the migration pathway for Ca2+. The effects of alanine mutations to Leu249 and Leu253 on Ca2+ dissociation may arise from destabilization of the hydrophobic wall lining the pathway. In mutant Lys252Glu, unfavorable interaction between the glutamate and L6-7 may open the pathway. In addition, Leu253Ala, and to a lesser extent some of the other mutations, reduced the rate of the E1PCa2 to E2P transition of the phosphoenzyme, enhanced the rate of dephosphorylation of E2P, and reduced the apparent affinity for vanadate, suggesting interference with the conformational change of the phosphoenzyme and the function of the catalytic site in E2 and E2P.