The turn motif is a phosphorylation switch that regulates the binding of Hsp70 to protein kinase C

Protein kinase Cdelta as gonadotropin-releasing hormone target isoenzyme in the alphaT3-1 gonadotrope cell line

We investigated the kinetics of gonadotropin-releasing hormone (GnRH)-induced activation of the protein kinase C (PKC) delta isoform in alphaT3-1 gonadotrope cells. Results were evaluated in subcellular fractions and whole-cell lysates using specific antibodies recognizing either non- or (trans- and auto-)phosphorylated forms of the kinase at Thr505 and Ser643 residues modulating stability and/or activation of the enzyme. Under basal conditions, and in contrast to PKC epsilon, PKC delta was mainly associated with the membrane compartment. GnRH (10(-7)M) elicited further and rapid membrane translocation and time-dependent phosphorylation at both sites of PKC delta. The neuropeptide's effects did not show a refractory period after short but successive GnRH stimulation and were abolished by the GnRH antagonist, antide. Sustained GnRH stimulation (2-6 h) provoked rapid down-regulation of PKC delta. Antide, by inhibiting the initial processes (translocation, phosphorylation), counteracted the degradation of the enzyme. Proteolytic processing of PKC delta was shown to mainly involve proteasome activity. Indeed, specific proteasome inhibitors prevented GnRH-elicited kinase depletion and induced membrane accumulation of the enzyme in a phosphorylated (Thr505, Ser643) form. Thus, GnRH may regulate time-dependent cell responses by modulating the phosphorylation/activation state of its signal transduction effector proteins, and by maintaining their appropriate expression balance via proteolytic processes involving the proteasome system.

Interaction between Src and a C-terminal proline-rich motif of Akt is required for Akt activation

Activation of Akt by growth factors is a multistep process. Here, we provide evidence that tyrosine kinase Src is directly associated with Akt through the interaction between its SH3 domain and a conserved proline-rich motif (PXXP) in the C-terminal regulatory region of Akt. Substitution of the proline residues Pro-424 and Pro-427 by alanines results in loss of Akt activity and phosphorylation induced by the epidermal growth factor (EGF), possibly because these mutations disrupt the interaction between Akt and the SH3 domain of Src. This possibility is corroborated by our observation that the Akt mutant lacking these two prolines fails to bind to Src both in vivo and in vitro. We also showed that phosphorylation of Tyr-315 in Akt induced by Src or EGF is dependent on the integrity of this proline-rich motif. Furthermore, the Akt mutant lacking this proline motif fails to block the transcription activity of Forkhead in 293 cells and poorly stimulates the proliferation of Madin-Darby canine kidney cells. Taken together, our data suggest that the interaction between the SH3 domain of Src family kinases and the proline-rich motif in the C-terminal regulatory region of Akt is required for tyrosine phosphorylation of Akt and its subsequent activation. It is noteworthy that this PXXP motif is conserved throughout several members of AGC kinase family, implying that association of this motif with the SH3 domain of an upstream regulator may represent a general mechanism applicable to these kinases as well.

Regulation of the ABC kinases by phosphorylation: protein kinase C as a paradigm

Phosphorylation plays a central role in regulating the activation and signalling lifetime of protein kinases A, B (also known as Akt) and C. These kinases share three conserved phosphorylation motifs: the activation loop segment, the turn motif and the hydrophobic motif. This review focuses on how phosphorylation at each of these sites regulates the maturation, signalling and down-regulation of PKC as a paradigm for how these sites control the function of the ABC kinases.

High glucose and insulin promote O-GlcNAc modification of proteins, including alpha-tubulin

Increased flux through the hexosamine biosynthesis pathway has been implicated in the development of glucose-induced insulin resistance and may promote the modification of certain proteins with O-linked N-acetylglucosamine (O-GlcNAc). L6 myotubes (a model of skeletal muscle) were incubated for 18 h in 5 or 25 mM glucose with or without 10 nM insulin. As assessed by immunoblotting with an O-GlcNAc-specific antibody, high glucose and/or insulin enhanced O-GlcNAcylation of numerous proteins, including the transcription factor Sp1, a known substrate for this modification. To identify novel proteins that may be O-GlcNAc modified in a glucose concentration/insulin-responsive manner, total cell membranes were separated by one- or two-dimensional gel electrophoresis. Selected O-GlcNAcylated proteins were identified by mass spectrometry (MS) analysis. MS sequencing of tryptic peptides identified member(s) of the heat shock protein 70 (HSP70) family and rat alpha-tubulin. Immunoprecipitation/immunoblot studies demonstrated several HSP70 isoforms and/or posttranslational modifications, some with selectively enhanced O-GlcNAcylation following exposure to high glucose plus insulin. In conclusion, in L6 myotubes, Sp1, membrane-associated HSP70, and alpha-tubulin are O-GlcNAcylated; the modification is markedly enhanced by sustained increased glucose flux.