Phosphorylation of the C-terminal domain of the Na+/H+ exchanger by Ca2+/calmodulin-dependent protein kinase II.
The Na+/H+ exchanger is a pH-regulatory protein that extrudes one H+ ion in exchange for one Na+ ion when intracellular pH declines. A number of studies have shown phorbol ester stimulation of activity in intact cells, leading to the idea that the exchanger is regulated by protein kinase C-mediated phosphorylation in vivo. cDNA encoding the protein has been cloned, and a recent model suggests a large internal cytoplasmic C-terminal domain that may be a site of regulation of the exchanger [Sardet, Franchi & Pouyssegur (1989) Cell 56, 271-280]. We examined this region of the protein using a rabbit cardiac Na+/H+ exchanger cDNA clone. cDNA of the Na+/H+ exchanger, coding for the C-terminal 178 amino acid residues, was cloned into the expression vector pEX-1 and expressed as a fusion protein with beta-galactosidase. The fusion protein reacted with an antibody produced against a synthetic peptide of the C-terminal 13 amino acid residues of the Na+/H+ exchanger, confirming the identity of the expressed protein. Control and experimental pEX-1-Na+/H+ exchanger protein was purified on a p-aminophenyl beta-D-thiogalactopyranoside-agarose column. Purified Ca2+/calmodulin-dependent protein kinase II readily phosphorylated the Na+/H+ exchanger protein in a Ca(2+)- and calmodulin-dependent manner in vitro, but this region of the protein was not a substrate for purified protein kinase C or for the catalytic subunit of cyclic AMP-dependent protein kinase. Control-expressed beta-galactosidase was phosphorylated to a maximal level of 0.77 +/- 0.17 mol of Pi/mol (mean +/- S.E.M., n = 6) whereas the fusion protein was phosphorylated to a maximal level of 4.09 +/- 0.39 mol of Pi/mol (n = 6), suggesting one site of phosphorylation in beta-galactosidase and three in the C-terminal domain of the Na+/H+ exchanger. Examination of the deduced amino acid sequence of this part of the exchanger reveals three consensus sequences for Ca2+/calmodulin-dependent protein kinase II. These results suggest that the exchanger may be directly regulated in vivo by calmodulin-dependent protein kinase II but not by protein kinase C or cyclic AMP-dependent protein kinase.
Fliegel
L
L
Department of Biochemistry and Pediatrics, Faculty of Medicine, University of Alberta, Edmonton, Canada.
Walsh
M P
MP
Singh
D
D
Wong
C
C
Barr
A
A
D000595:Amino Acid Sequence; D000818:Animals; D001665:Binding Sites; D017871:Calcium-Calmodulin-Dependent Protein Kinases; D002352:Carrier Proteins / Q000235:genetics / Q000302:isolation & purification / Q000378:metabolism*; D003001:Cloning, Molecular; D004591:Electrophoresis, Polyacrylamide Gel; D004926:Escherichia coli / Q000235:genetics; D007700:Kinetics; D008969:Molecular Sequence Data; D009206:Myocardium / Q000378:metabolism; D010766:Phosphorylation; D011494:Protein Kinases / Q000378:metabolism*; D011817:Rabbits; D011993:Recombinant Fusion Proteins / Q000302:isolation & purification / Q000378:metabolism; D011994:Recombinant Proteins / Q000302:isolation & purification / Q000378:metabolism; D017923:Sodium-Hydrogen Exchangers; D001616:beta-Galactosidase / Q000235:genetics / Q000302:isolation & purification / Q000378:metabolism
D002352:Carrier Proteins; D011993:Recombinant Fusion Proteins; D011994:Recombinant Proteins; D017923:Sodium-Hydrogen Exchangers; D011494:Protein Kinases; D017871:Calcium-Calmodulin-Dependent Protein Kinases; D001616:beta-Galactosidase