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Akt (= protein kinase B), a subfamily of the AGC serine/threonine kinases, plays critical roles in survival, proliferation, glucose metabolism, and other cellular functions. Akt activation requires the recruitment of the enzyme to the plasma membrane by interacting with membrane-bound lipid products of phosphatidylinositol 3-kinase. Membrane-bound Akt is then phosphorylated at two sites for its full activation; Thr-308 in the activation loop of the kinase domain is phosphorylated by 3-phosphoinositide-dependent kinase-1 (PDK1) and Ser-473 in the C-terminal hydrophobic motif by a putative kinase PDK2. The identity of PDK2 has been elusive. Here we present evidence that conventional isoforms of protein kinase C (PKC), particularly PKCbetaII, can regulate Akt activity by directly phosphorylating Ser-473 in vitro and in IgE/antigen-stimulated mast cells. By contrast, PKCbeta is not required for Ser-473 phosphorylation in mast cells stimulated with stem cell factor or interleukin-3, in serum-stimulated fibroblasts, or in antigen receptor-stimulated T or B lymphocytes. Therefore, PKCbetaII appears to work as a cell type- and stimulus-specific PDK2.

Protein kinase C (PKC) and Syk protein tyrosine kinase play critical roles in immune cell activation including that through the high-affinity IgE receptor, FcepsilonRI. Mechanisms by which PKC activation leads to the activation of Ras, a family of GTPases essential for immune cell activation, have been elusive. We present evidence that Tyr-662 and Tyr-658 of PKCbetaI and PKCalpha, respectively, are phosphorylated by Syk in the membrane compartment of FcepsilonRI-stimulated mast cells. These phosphorylations require prior PKC autophosphorylation of the adjacent serine residues (Ser-661 and Ser-657, respectively) and generate a binding site for the SH2 domain of the adaptor protein Grb-2. By recruiting the Grb-2/Sos complex to the plasma membrane, these conventional PKC isoforms contribute to the full activation of the Ras/extracellular signal-regulated kinase signaling pathway in FcepsilonRI-stimulated mast cells.

PKC beta I and PKC beta II are DAG- and Ca(2+)-dependent conventional or classical isoforms of protein kinase C. Generated by alternative splicing from a single gene, they differ at their C-terminal 50 (beta I) or 52 (beta II) residues. They are expressed as major PKC isoforms in a variety of tissues, and thus the functions ascribed to "PKC" based on early studies using phorbol esters and PKC inhibitors could be attributed to them. As tools to probe into isoform-specific functions have recently become available, our understanding of the normal functions of these isoforms has dramatically increased. This minireview will focus mainly on two areas of signal transduction where the roles of PKC beta I and PKC beta II are relatively well-characterized: immunoreceptor and insulin receptor systems. Their involvement in disorders due to pertubations in these signaling systems, i.e., immunodeficiencies and diabetes, is also reviewed. Finally, patterns of PKC action in these and other biologic systems are discussed.

PKCβI and PKCβII are DAG- and Ca2+-dependent conventional or classical isoforms of protein kinase C. Generated by alternative splicing from a single gene, they differ at their C-terminal 50 (βI) or 52 (βII) residues. They are expressed as major PKC isoforms in a variety of tissues, and thus the functions ascribed to “PKC” based on early studies using phorbol esters and PKC inhibitors could be attributed to them. As tools to probe into isoform-specific functions have recently become available, our understanding of the normal functions of these isoforms has dramatically increased. This minireview will focus mainly on two areas of signal transduction where the roles of PKCβI and PKCβII are relatively well-characterized: immunoreceptor and insulin receptor systems. Their involvement in disorders due to pertubations in these signaling systems, i.e., immunodeficiencies and diabetes, is also reviewed. Finally, patterns of PKC action in these and other biologic systems are discussed. [ABSTRACT FROM AUTHOR]

Mutations in Bruton's tyrosine kinase (Btk) result in X-linked agammaglobulinemia (XLA) in humans and X-linked immunodeficiency (xid) in mice. While targeted disruption of the protein kinase C-β (PKCβ) gene in mice results in an immunodeficiency similar to xid, the overall tyrosine phosphorylation of Btk is significantly enhanced in PKCβ-deficient B cells. We provide direct evidence that PKCβ acts as a feedback loop inhibitor of Btk activation. Inhibition of PKCβ results in a dramatic increase in B-cell receptor (BCR)-mediated Ca2+ signaling. We identified a highly conserved PKCβ serine phosphorylation site in a short linker within the Tec homology domain of Btk. Mutation of this phosphorylation site led to enhanced tyrosine phosphorylation and membrane association of Btk, and augmented BCR and Fc∊RI-mediated signaling in B and mast cells, respectively. These findings provide a novel mechanism whereby reversible translocation of Btk/Tec kinases regulates the threshold for immunoreceptor signaling and thereby modulates lymphocyte activation. [ABSTRACT FROM AUTHOR]

Describes a study which provides genetic, biochemical and pharmacological evidence that, on high-affinity immunoglobulin receptors stimulation, Syk regulates Bruton's tyrosine kinase (Btk), and Btk regulates the membrane translocation and enzymic activity of protein kinase C. Cell culture and stimulation; Immunoblotting analysis and antibodies; Results and discussion.