Protein kinase C isotypes and signal-transduction in human neutrophils: Selective substrate specificity of calcium-dependent β-PKC and novel calcium-independent nPKC

Abstract

Neutrophils possess at least two phospholipid-dependent forms of protein kinase C, a classical Ca/PS/DG-dependent β-isotype of protein kinase C and a Ca-independent but PS/DG-dependent novel protein kinase C (nPKC) which we now demonstrate to have different substrate specificities. Activation of human neutrophils triggers assembly of an NADPH oxidase in the membrane and generation of O − 2. A role for the major Ca-dependent isotype β-PKC in neutrophils is proposed in stimulus-induced phosphorylation and association of a cytosolic 47 kDa protein (p47-phox) with the membrane NADPH oxidase. In this study we demonstrate that purified α-PKC and nPKC have very different substrate specificities; β-PKC but not nPKC phosphorylated both endogenous and recombinant p47-phox. In addition, β-PKC but not nPKC phosphorylated [ser 25]PKC(19–31), the substrate peptide based on a sequence in the Ca-dependent α, β and γ-isotypes. Pscudosubstrate(19–36), derived from the C-terminus of Ca-dependent PKC isotypes, inhibited β-PKC but not nPKC activity using either Histone IIIS or peptide(19–31) as substrate. Pseudosubstrate(19–36) also inhibited β-PKC catalyzed phosphorylation of endogenous and recombinant p47-phox. Pseudosubstrate(19–36) inhibited the O − 2 generation triggered by GTPγS in electroporated neutrophils by 50%. 32P-Labelled neutrophils electroporated in the presence of GTPγS showed phosphorylation of multiple cytosolic proteins including a 47 kDa band, and phosphorylation of membrane-associated 34 kDa, 47 kDa and 54 kDa proteins. Pseudosubstrate(19–36) inhibited phosphorylation of p47-phox in the membrane but not in the cytosol. These findings suggest translocatable, Ca-dependent isotypes of PKC such as β-PKC may play a role in the phosphorylation of membrane associated p47-phox and the assembly or maintenance of an active NADPH oxidase.