title: Phosphorylation by protein kinase C and cyclic AMP-dependent protein kinase of synthetic peptides derived from the linker region of human P-glycoprotein.
abstract: Specific sites in the linker region of human P-glycoprotein phosphorylated by protein kinase C (PKC) were identified by means of a synthetic peptide substrate, PG-2, corresponding to residues 656-689 from this region of the molecule. As PG-2 has several sequences of the type recognized by the cyclic AMP-dependent protein kinase (PKA), PG-2 was also tested as a substrate for PKA. PG-2 was phosphorylated by purified PKC in a Ca2+/phospholipid-dependent manner, with a Km of 1.3 microM, and to a maximum stoichiometry of 2.9 +/- 0.1 mol of phosphate/mol of peptide. Sequence analysis of tryptic fragments of PG-2 phosphorylated by PKC identified Ser-661, Ser-667 and Ser-671 as the three sites of phosphorylation. PG-2 was also found to be phosphorylated by purified PKA in a cyclic AMP-dependent manner, with a Km of 21 microM, and to a maximum stoichiometry of 2.6 +/- 0.2 mol of phosphate/mol of peptide. Ser-667, Ser-671 and Ser-683 were phosphorylated by PKA. Truncated peptides of PG-2 were utilized to confirm that Ser-661 was PKC-specific and Ser-683 was PKA-specific. Further studies showed that PG-2 acted as a competitive substrate for the P-glycoprotein kinase present in membranes from multidrug-resistant human KB cells. The membrane kinase phosphorylated PG-2 mainly on Ser-661, Ser-667 and Ser-671. These results show that human P-glycoprotein can be phosphorylated by at least two protein kinases, stimulated by different second-messenger systems, which exhibit both overlapping and unique specificities for phosphorylation of multiple sites in the linker region of the molecule.
authors.affiliation: Department of Pharmacology, Emory University School of Medicine, Altanta, GA 30322.
authors.forename: T C
authors.initials: TC
authors.lastname: Chambers
authors.forename: J
authors.initials: J
authors.lastname: Pohl
authors.forename: D B
authors.initials: DB
authors.lastname: Glass
authors.forename: J F
authors.initials: JF
authors.lastname: Kuo
mesh_terms: D020168:ATP Binding Cassette Transporter, Subfamily B, Member 1; D000595:Amino Acid Sequence; D002352:Carrier Proteins / Q000138:chemical synthesis / Q000378:metabolism*; D002462:Cell Membrane / Q000201:enzymology; D002478:Cells, Cultured; D017868:Cyclic AMP-Dependent Protein Kinases / Q000378:metabolism*; D004351:Drug Resistance; D006801:Humans; D008562:Membrane Glycoproteins / Q000138:chemical synthesis / Q000378:metabolism*; D008969:Molecular Sequence Data; D010446:Peptide Fragments / Q000138:chemical synthesis / Q000378:metabolism; D010449:Peptide Mapping; D010766:Phosphorylation; D011493:Protein Kinase C / Q000378:metabolism*; D013379:Substrate Specificity
chemical_list: D020168:ATP Binding Cassette Transporter, Subfamily B, Member 1; D002352:Carrier Proteins; D008562:Membrane Glycoproteins; D010446:Peptide Fragments; D017868:Cyclic AMP-Dependent Protein Kinases; D011493:Protein Kinase C