PKCmu, Active

Protein kinase Cmu (PKCmu) is a novel member of the protein kinase C (PKC) family that differs from the other isoenzymes in structural and enzymatic properties. It is characterized by the presence of a pleckstrin homology (PH) domain and an amino-terminal hydrophobic region and has substrate specificity distinct from other PKC isoforms. PKCmu is a ubiquitous PKC isotype with the highest expression in the thymus, lung and peripheral blood mononuclear cells (1). PKCmu forms a complex in vivo with a phosphatidylinositol 4-kinase and a phosphatidylinositol-4-phosphate 5-kinase. A region of PKCmu between the amino-terminal transmembrane domain and the pleckstrin homology domain is shown to be involved in the association with the lipid kinases (2). PKCmu was also shown to associate with the B cell receptor (BCR) complex and its activity is up-regulated after cross-linking the BCR and CD19 on B cells (3). PKC mu co-precipitates with Syk and phospholipase C-γ 1/2 (PLC γ 1/2) and in vitro phosphorylation of fusion proteins showed that both Syk and PLC γ 1 are potential substrates of PKC mu in vivo. In addition, specific interaction of PKCmu and 14-3-3tau can be shown in the T cell line Jurkat by immunocoprecipitiation and by pulldown assays (4). 14-3-3tau is not a substrate of PKCmu and strongly down-regulates PKCmu kinase activity in vitro. In response to various stimuli, PKC mu activates the mitogen-activated protein kinase (p42/ERK1 MAPK cascade) but does not affect the related c-jun N-terminal kinase or p38 MAPK (5).