Evidence that a G-protein transduces signals initiated by the protein-tyrosine kinase v-Fps

Divergence in the G-protein-coupled receptor mitogenic signalling pathway at the level of Raf kinase

While activation of tyrosine kinase growth factor receptors is accompanied by hyperphosphorylation of Raf-1, stimulation of receptors coupled to G-proteins has in some cases been shown to result in activation of a non-Raf MEKK rather than of Raf itself. Our finding (Weiss, R. H., and Nuccitelli, R. (1992) J. Biol. Chem. 267, 5608-5613) that the thrombin receptor requires tyrosine phosphorylation for its mitogenic effect in vascular smooth muscle cells led us to search for the molecules which are being tyrosine phosphorylated by this receptor. To determine whether mitogenic signalling of G-protein-coupled growth factor receptors results in tyrosine phosphorylation of Raf, we examined activation of Raf by two such receptors. Both thrombin and angiotensin II are mitogenic in NIH3T3 cells, but only thrombin causes hyperphosphorylation of Raf-1. Activation of Raf by thrombin is associated with phosphorylation of Raf-1 on tyrosine residues, whereas activation of Raf by angiotensin II does not involve significant tyrosine phosphorylation. However, Shc is tyrosine phosphorylated by both thrombin and angiotensin II. Thus, there exists a divergence in the mitogenic signalling pathways of the G-protein-coupled receptors relative to the Raf signalling cascade. While both thrombin and angiotensin II phosphorylate Shc and activate Raf catalytic activity, only thrombin phosphorylated Raf-1 on tyrosine. This signalling through disparate Raf-coupled pathways suggests one means by which the G-protein-coupled receptors may confer specificity in their signalling properties.

Interaction between G proteins and tyrosine kinases upon T cell receptor.CD3-mediated signaling

Engagement of the T cell receptor (TCR).CD3 complex results in the induction of multiple intracellular events, with protein tyrosine kinases playing a pivotal role in their initiation. Biochemical studies also exist suggesting the involvement of heterotrimeric GTP-binding proteins (G proteins); however, the functional consequence of this participation in TCR.CD3-mediated signaling is unresolved. Here, we report TCR.CD3-mediated guanine nucleotide exchange among the 42-kDa G protein alpha subunits of the G alpha q/11 family, their physical association with CD3 epsilon, and the G alpha 11-dependent activation of phospholipase C beta. Protein tyrosine kinase inhibitors, however, abrogate TCR.CD3-mediated G protein activation. Quite interesting is the observation that cells transfected with a function-deficient mutant of G alpha 11 display diminished tyrosine phosphorylation of TCR.CD3 zeta and epsilon chains, as well as ZAP-70, upon anti-CD3 antibody triggering. These data indicate the involvement of the G alpha q/11 family in TCR.CD3 signaling at a step proximal to the receptor and suggest a reciprocal regulation between tyrosine kinases and G proteins in T cells.

Study of the phorbol ester effect on Alzheimer amyloid precursor processing: sequence requirements and involvement of a cholera toxin sensitive protein

Phorbol esters (PDBu) stimulate alpha-secretase cleavage and secretion of the Alzheimer amyloid precursor protein (APP). To determine whether any cytoplasmic residues or sequence motifs mediate the PDBu effect on APP processing, this region of APP was altered by point mutations or deletions. To differentiate the mutated APP from the endogenous APP, the APP751 ectodomain between amino acids 1 and 647 was replaced by a human secreted alkaline phosphatase derivative (SEAP). The resultant fusion protein (SEAP-APP751) was cleaved by alpha-secretase at the same site as full-length APP, and its secretion was stimulated by PDBu at a level similar to APP751. However, PDBu-stimulated secretion of the SEAP-APP751 fusion protein reached its maximum level after 30 min of treatment, while secretion of APP751 reached its maximum after 60 min, suggesting that the APP ectodomain affects the kinetics of APP secretion. Mutation of the cytoplasmic serines to alanines had no effect on the PDBu-stimulated secretion of the SEAP-APP, indicating that protein kinase C (PKC) phosphorylation of the cytoplasmic domain of APP is not important for stimulation of APP secretion. Similarly, deletion of the cytoplasmic domain between amino acids 719 and 751 had no effect on the PDBu-stimulated secretion. However, deletion of amino acids 707-751 resulted in a significant increase in the secretory cleavage of the SEAP-APP707 delta C construct, suggesting that the sequence 707-719 is important for the regulated secretion of APP. Cholera toxin, but not pertussis toxin, reduced the PDBu-induced secretion of APP by more than two-fold, suggesting that the PDBu response may be modulated by a cholera toxin sensitive heterotrimeric G-protein.

The role of G-proteins versus protein tyrosine kinases in the regulation of lymphocyte activation

The relative roles of G-proteins and protein tyrosine kinases (PTKs) in the regulation of antigen receptor-mediated signalling in B and T cells is controversial. As they, and the biochemical events they control, are potential targets for intervention in various immune dysfunctions, the resolution of the controversy is of great interest. Here, Margaret Harnett and Kevin Rigley provide a timely assessment of current understanding, and propose a model for the interaction of G-proteins and PTKs in antigen receptor-mediated signalling.

Sequence alignment of the G-protein coupled receptor superfamily

The multitude of G-protein coupled receptor (GPR) superfamily cDNAs recently isolated has exceeded the number of receptor subtypes anticipated by pharmacological studies. Analysis of the sequence similarities and unique features of the members of this family is valuable for designing strategies to isolate related cDNAs, for developing hypotheses concerning substrate-ligand and receptor-effector interactions, and for understanding the evolution of these genes. We have compiled and aligned the 74 unique amino acid sequences published to date and review the present understanding of the structural motifs contributing to ligand binding and G-protein coupling.