Differential phosphorylation of sites in the linker region of P-glycoprotein by protein kinase C isozymes α, βi, βii, γ, δ, ϵ, η, and ζ

Abstract

To determine whether individual protein kinase C (PKC) isozymes differentially phosphorylate sites in the linker region of human P-glycoprotein (P-gp), we used a synthetic peptide substrate, PG-2, exactly corresponding to amino acid residues spanning the region 656–689 of the multidrug resistance gene ( MDR1). All tested PKC isozymes phosphorylated PG-2. The maximum phosphate incorporation by calcium-dependent PKC isozymes α, βI, βII, and γ was 3, 2, 2, and 3 mol phosphate/mol PG-2, respectively. The maximum phosphate incorporation by calcium-independent isozymes δ, ϵ, η, and ζ was 1.5, 0.5, 1.5, and 1.5 mol phosphate/mol PG-2, respectively. Two-dimensional tryptic phosphopeptide mapping indicated differential phosphorylation of the PKC consensus sites Ser-661, Ser-667, and Ser-671 by individual isozymes, which may be functionally significant. These data suggest that differential phosphorylation by PKC isoenzymes of PKC sites within the P-gp linker region may play a role in modulating P-gp activity.