Autoinhibitory domains of various Ca2+ transporters cross-react (original) (raw)
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The Biochemical journal, 1998
A mutant of the plasma membrane Ca2+ pump (PMCA) called (nCI)hPMCA4b(ct120), in which the C-terminal regulatory segment including the calmodulin-binding autoinhibitory domains C and I had been relocated near the N-terminus, has been expressed in COS-1 cells. The measurements of Ca2+ transport in microsomal preparations showed that the rearranged enzyme was functional. The activity of the (nCI)hPMCA4b(ct120) mutant was compared with those of the wild-type hPMCA4b and the fully active calmodulin-insensitive mutant hPMCA4b(ct120). In the absence of calmodulin the activity of (nCI)hPMCA4b(ct120) was higher than that of hPMCA4b but only 45% of that of hPMCA4b(ct120). Mutant (nCI)hPMCA4b(ct120) exhibited an apparent affinity for Ca2+ similar to that of hPMCA4b, typical of the inhibited state of the enzyme. Calmodulin at concentrations that fully activated hPMCA4b increased the activity of (nCI)hPMCA4b(ct120) to 68% of that of hPMCA4b(ct120). The lower maximal activity of (nCI)hPMCA4b(ct12...
Plasma Membrane Ca2+ Pump Isoforms 2a and 2b Are Unusually Responsive to Calmodulin and Ca2+
Journal of Biological Chemistry, 1997
The full-length a and b variants of the rat plasma membrane calcium pump, isoform 2 (rPMCA2a and rPMCA2b), were constructed and expressed in COS-7 cells. To characterize these isoforms, calcium transport was determined in a microsomal fraction. Both rPMCA2a and rPMCA2b had a much higher affinity for calmodulin than the corresponding forms of hPMCA4, and rPMCA2b had the highest affinity among the isoforms that have been tested so far. When analyzed at a relatively high calmodulin concentration, rPMCA2b and, to a lesser extent, rPMCA2a showed higher apparent calcium affinity; i.e. they were more active at lower Ca 2؉ concentrations than hPMCA4b. This indicates that these two variants of rat isoform 2 will tend to maintain a lower free cytosolic Ca 2؉ level in cells where they are expressed. Both variants also showed a higher level of basal activity (in the complete absence of calmodulin) than hPMCA4b, a property which would reinforce their ability to maintain a low free cytosolic Ca 2؉ concentration. Experiments designed to determine the source of the higher apparent Ca 2؉ affinity of rPMCA2b showed that it came from the properties of the carboxyl terminus, rather than from any difference in the catalytic core.
Journal of Biological Chemistry, 2007
The inhibition by the regulatory domain and the interaction with calmodulin (CaM) vary among plasma membrane calcium pump (PMCA) isoforms. To explore these differences, the kinetics of CaM effects on PMCA4a were investigated and compared with those of PMCA4b. The maximal apparent rate constant for CaM activation of PMCA4a was almost twice that for PMCA4b, whereas the rates of activation for both isoforms showed similar dependence on Ca 2؉. The inactivation of PMCA4a by CaM removal was also faster than for PMCA4b, and Ca 2؉ showed a much smaller effect (2versus 30-fold modification). The rate constants of the individual steps that determine the overall rates were obtained from stopped-flow experiments in which binding of TA-CaM was observed by changes in its fluorescence. TA-CaM binds to two conformations of PMCA4a, an "open" conformation with high activity, and a "closed" one with lower activity. Compared with PMCA4b (
Interaction of calmodulin with the calmodulin binding domain of the plasma membrane calcium pump
Biochemistry, 1990
Peptides corresponding to the calmodulin binding domain of the plasma membrane Ca2+ pump (James et al., 1988) were synthesized, and their interaction with calmodulin was studied with circular dichroism, infrared spectroscopy, nuclear magnetic resonance, and fluorescence techniques. They corresponded to the complete calmodulin binding domain (28 residues), to its first 15 or 20 amino acids, and to its C-terminal 14 amino acids. The first three peptides interacted with calmodulin. The K value was similar to that of the intact enzyme in the 28 and 20 amino acid peptides, but increased substantially in the shorter 15 amino acid peptide. The 14 amino acid peptide corresponding to the C-terminal portion of the domain failed to bind calmodulin. 2D N M R experiments on the 20 amino acid peptides have indicated that the interaction occurred with the C-terminal half of calmodulin. A tryptophan that is conserved in most calmodulin binding 'This work has been made possible by the financial contributions of the Swiss Nationalfonds (Grant 3.53 1.086) and the National Institues of Health (Grant GM28835).
Journal of Biological Chemistry, 2007
The inhibition by the regulatory domain and the interaction with calmodulin (CaM) vary among plasma membrane calcium pump (PMCA) isoforms. To explore these differences, the kinetics of CaM effects on PMCA4a were investigated and compared with those of PMCA4b. The maximal apparent rate constant for CaM activation of PMCA4a was almost twice that for PMCA4b, whereas the rates of activation for both isoforms showed similar dependence on Ca 2؉. The inactivation of PMCA4a by CaM removal was also faster than for PMCA4b, and Ca 2؉ showed a much smaller effect (2versus 30-fold modification). The rate constants of the individual steps that determine the overall rates were obtained from stopped-flow experiments in which binding of TA-CaM was observed by changes in its fluorescence. TA-CaM binds to two conformations of PMCA4a, an "open" conformation with high activity, and a "closed" one with lower activity. Compared with PMCA4b (
Journal of Biological Chemistry, 1997
Phosphorylation by protein kinase C of the "a" and "b" variants of plasma membrane Ca 2؉ pump isoforms 2 and 3 was studied. Full-length versions of these isoforms were assembled and expressed in COS cells. Whereas the "a" forms were phosphorylated easily with PKC, isoform 2b was phosphorylated only a little, and isoform 3b was not phosphorylated at all. Phosphorylation of isoforms 2a and 3a did not affect their basal activity, but prevented the stimulation of their activity by calmodulin and their binding to calmodulin-Sepharose. This indicated that phosphorylation prevented activation of these isoforms by preventing calmodulin binding. Based on these results, phosphorylation of the pump with PKC would be expected to increase free intracellular Ca 2؉ levels in those cells where isoforms 2a and 3a are expressed.
Journal of Biological Chemistry, 1996
The carboxyl-terminal region of the plasma membrane Ca 2؉ pump isoform 4b contains two autoinhibitory regions which keep the pump inactive in the absence of activators such as calmodulin. One of these regions is approximately coterminous with the calmodulin-binding domain, while the second region is downstream (Verma, A. K., Enyedi, A., Filoteo, A. G., and Penniston, J. T. (1994) J. Biol. Chem. 269, 1687-1691). The carboxyl-terminal region has also been identified as the site for phosphorylation of this isoform by protein kinase C (1991) J. Biol. Chem. 266, 9078 -9085). Using constructs lacking various numbers of residues at the carboxyl terminus, we studied the degree of phosphorylation by protein kinase C and the resultant activation of Ca 2؉ transport. The results showed that the most specific and easy phosphorylation occurred in a region of about 20 residues which is downstream of the calmodulin-binding domain, and that the downstream inhibitory domain had also about the same size and location. Phosphorylation partially activated the pump by removing only the inhibition due to this region. Binding of calmodulin to the calmodulin-binding domain activated the pump more fully by removing the inhibition due to both regions, regardless of the state of phosphorylation at the downstream inhibitory region.