Tohru Kozasa

THE GTPASE-ACTIVATING PROTEIN RGS4 STABILIZES THE TRANSITION STATE FOR NUCLEOTIDE HYDROLYSIS

RGS proteins constitute a newly appreciated group of negative regulators of G protein signaling. Discovered by genetic screens in yeast, worms, and other organisms, two mammalian RGS proteins, RGS4 and GAIP, act as GTPase-activating proteins for members of the Gi family of G protein α subunits. We have purified recombinant RGS4 to homogeneity and demonstrate that it acts catalytically to stimulate GTP hydrolysis by Gi proteins. Furthermore, RGS4 stabilizes the transition state for GTP hydrolysis, as evidenced by its high affinity for the GDP-AlF4−-bound forms of Goα and Giα and its relatively low affinity for the GTPγS- and GDP-bound forms of these proteins. Consequently, RGS4 is most likely not a downstream effector for activated Gα subunits. All members of the Gi subfamily of proteins tested are substrates for RGS4 (including Gtα and Gzα); the protein has lower affinity for Gqα, and it does not stimulate the GTPase activity of Gsα or G12α.

SELECTIVE REGULATION OF GALPHA Q/11 BY AN RGS DOMAIN IN THE G PROTEIN-COUPLED RECEPTOR KINASE, GRK2

G protein-coupled receptor kinases (GRKs) are well characterized regulators of G protein-coupled receptors, whereas regulators of G protein signaling (RGS) proteins directly control the activity of G protein α subunits. Interestingly, a recent report (Siderovski, D. P., Hessel, A., Chung, S., Mak, T. W., and Tyers, M. (1996) Curr. Biol. 6, 211–212) identified a region within the N terminus of GRKs that contained homology to RGS domains. Given that RGS domains demonstrate AlF4 −-dependent binding to G protein α subunits, we tested the ability of G proteins from a crude bovine brain extract to bind to GRK affinity columns in the absence or presence of AlF4 −. This revealed the specific ability of bovine brain Gαq/11 to bind to both GRK2 and GRK3 in an AlF4 −-dependent manner. In contrast, Gαs, Gαi, and Gα12/13 did not bind to GRK2 or GRK3 despite their presence in the extract. Additional studies revealed that bovine brain Gαq/11 could also bind to an N-terminal construct of GRK2, while no binding of Gαq/11, Gαs, Gαi, or Gα12/13 to comparable constructs of GRK5 or GRK6 was observed. Experiments using purified Gαq revealed significant binding of both GαqGDP/AlF4 − and Gαq(GTPγS), but not Gαq(GDP), to GRK2. Activation-dependent binding was also observed in both COS-1 and HEK293 cells as GRK2 significantly co-immunoprecipitated constitutively active Gαq(R183C) but not wild type Gαq. In vitro analysis revealed that GRK2 possesses weak GAP activity toward Gαq that is dependent on the presence of a G protein-coupled receptor. However, GRK2 effectively inhibited Gαq-mediated activation of phospholipase C-β both in vitro and in cells, possibly through sequestration of activated Gαq. These data suggest that a subfamily of the GRKs may be bifunctional regulators of G protein-coupled receptor signaling operating directly on both receptors and G proteins.

Role of a ras homolog in the life cycle of schizosaccharomyces pombe

We have analyzed the function of the only ras homolog in S. pombe detectable by Southern blotting, ras1, which is homologous to mammalian ras genes and has been cloned. We have disrupted the ras1 gene and have replaced it with ras1Val17, which corresponds to a transforming variant of mammalian ras. Loss of ras1 activity by disruption results in the complete inability to mate. The cell body of a ras1- strain is extensively deformed, and a ras1-/ras1- diploid sporulates very poorly. Unlike RAS1 and RAS2 of S. cerevisiae, ras1 of S. pombe appears to have no effect on adenylate cyclase activity. This suggests that the target enzymes presumably modulated by ras proteins in signal transduction are not the same for all organisms.

Gα12 activates Rho GTPase through tyrosine-phosphorylated leukemia-associated RhoGEF

Heterotrimeric G proteins, G12 and G13, have been shown to transduce signals from G protein-coupled receptors to activate Rho GTPase in cells. Recently, we identified p115RhoGEF, one of the guanine nucleotide exchange factors (GEFs) for Rho, as a direct link between Gα13 and Rho [Kozasa, T., et al. (1998) Science 280, 2109–2111; Hart, M. J., et al. (1998) Science 280, 2112–2114]. Activated Gα13 stimulated the RhoGEF activity of p115 through interaction with the N-terminal RGS domain. However, Gα12 could not activate Rho through p115, although it interacted with the RGS domain of p115. The biochemical mechanism from Gα12 to Rho activation remained unknown. In this study, we analyzed the interaction of leukemia-associated RhoGEF (LARG), which also contains RGS domain, with Gα12 and Gα13. RGS domain of LARG demonstrated Gα12- and Gα13-specific GAP activity. LARG synergistically stimulated SRF activation by Gα12 and Gα13 in HeLa cells, and the SRF activation by Gα12-LARG was further stimulated by coexpression of Tec tyrosine kinase. It was also found that LARG is phosphorylated on tyrosine by Tec. In reconstitution assays, the RhoGEF activity of nonphosphorylated LARG was stimulated by Gα13 but not Gα12. However, when LARG was phosphorylated by Tec, Gα12 effectively stimulated the RhoGEF activity of LARG. These results demonstrate the biochemical mechanism of Rho activation through Gα12 and that the regulation of RhoGEFs by heterotrimeric G proteins G12/13 is further modulated by tyrosine phosphorylation.

The G protein G-alpha-12 stimulates Bruton's tyrosine kinase and a rasGAP through a conserved PH/BM domain

Heterotrimeric guanine-nucleotide-binding proteins (G proteins) are signal transducers that relay messages from many receptors on the cell surface to modulate various cellular processes. The direct downstream effectors of G proteins consist of the signalling molecules that are activated by their physical interactions with a Gα or Gβγ subunit. Effectors that interact directly with Gα12 G proteins have yet to be identified,. Here we show that Gα12 binds directly to, and stimulates the activity of, Brutons tyrosine kinase (Btk) and a Ras GTPase-activating protein, Gap1m, in vitro and in vivo. Gα12 interacts with a conserved domain, composed of the pleckstrin-homology domain and the adjacent Btk motif, that is present in both Btk and Gap1m. Our results are, to our knowledge, the first to identify direct effectors for Gα12 and to show that there is a direct link between heterotrimeric and monomeric G proteins.

Direct stimulation of Bruton's tyrosine kinase by Gq-protein alpha-subunit

Heterotrimeric guanine-nucleotide-binding regulatory proteins (G proteins) transduce signals from a wide variety of cell-surface receptors to generate physiological responses. Protein-tyrosine kinases are another group of critical cellular signal transducers and their malfunction often leads to cancer. Although activation of G-protein-coupled receptors can elicit rapid stimulation of cellular protein-tyrosine phosphorylation, the mechanism used by G proteins to activate protein-tyrosine kinases is unclear. Here we show that the purified α-subunit of the Gq class of G proteins (Gαq) directly stimulates the activity of a purified non-receptor kinase, Brutons tyrosine kinase (Btk), whereas purified α-subunits from Gi1, GO or Gz proteins do not. Gαq can also activate Btk in vivo. Furthermore, in Btk-deficient cells, stimulation of another kinase, a p38 MAP kinase, by Gq-coupled receptors is blocked. Our results demonstrate that certain protein-tyrosine kinases can be direct effectors of G proteins.

Pericyte-specific expression of Rgs5: implications for PDGF and EDG receptor signaling during vascular maturation

RGS proteins finely tune heterotrimeric G‐protein signaling. Implying the need for such fine‐tuning in the developing vascular system, in situ hybridization revealed a striking and extensive expression pattern of Rgs5 in the arterial walls of E12.5–E17.5 mouse embryos. The distribution and location of the Rgs5‐positive cells typified that of pericytes and strikingly overlapped the known expression pattern of platelet‐derived growth factor receptor (PDGFR)‐β. Both E14.5 PDGFR‐β‐ and platelet‐derived growth factor (PDGF)‐B‐deficient mice exhibited markedly reduced levels of Rgs5 in their vascular plexa and small arteries. This likely reflects the loss of pericytes in the mutant mice. RGS5 acts as a potent GTPase activating protein for Giα and Gqα and it attenuated angiotensin II‐, endothelin‐1‐, sphingosine‐1‐phosphate‐, and PDGF‐induced ERK‐2 phosphorylation. Together these results indicate that RGS5 exerts control over PDGFR‐β and GPCR‐mediated signaling pathways active during fetal vascular maturation.

Snapshot of Activated G Proteins at the Membrane: The Gαq-GRK2-Gßγ Complex

G protein–coupled receptor kinase 2 (GRK2) plays a key role in the desensitization of G protein–coupled receptor signaling by phosphorylating activated heptahelical receptors and by sequestering heterotrimeric G proteins. We report the atomic structure of GRK2 in complex with Gαq and Gβγ, in which the activated Gα subunit of Gq is fully dissociated from Gβγ and dramatically reoriented from its position in the inactive Gαβγ heterotrimer. Gαq forms an effector-like interaction with the GRK2 regulator of G protein signaling (RGS) homology domain that is distinct from and does not overlap with that used to bind RGS proteins such as RGS4.

G Protein Subunit Gα13 Binds to Integrin αIIbβ3 and Mediates Integrin “Outside-In” Signaling

Integrin G Protein Adhesion molecules, known as integrins, are found on the surface of cells. When integrins adhere to components of the extracellular matrix, they act as receptors and initiate signaling events within the cell. Gong et al. (p. 340) show that they do so in part by partnering with a signal-transducing protein called Gα13. Such α subunits of heterotrimeric guanine nucleotide-binding proteins are well known for transducing signals from the large class of G protein–coupled receptors, but were not known to work with integrins. Gα13 appears to interact directly with the integrin αIIbβ3 and to transmit signals that regulate cell spreading. Cell adhesion mediated by integrins is coupled to intracellular signaling by direct binding to G proteins. Integrins mediate cell adhesion to the extracellular matrix and transmit signals within the cell that stimulate cell spreading, retraction, migration, and proliferation. The mechanism of integrin outside-in signaling has been unclear. We found that the heterotrimeric guanine nucleotide–binding protein (G protein) Gα13 directly bound to the integrin β3 cytoplasmic domain and that Gα13-integrin interaction was promoted by ligand binding to the integrin αIIbβ3 and by guanosine triphosphate (GTP) loading of Gα13. Interference of Gα13 expression or a myristoylated fragment of Gα13 that inhibited interaction of αIIbβ3 with Gα13 diminished activation of protein kinase c-Src and stimulated the small guanosine triphosphatase RhoA, consequently inhibiting cell spreading and accelerating cell retraction. We conclude that integrins are noncanonical Gα13-coupled receptors that provide a mechanism for dynamic regulation of RhoA.

RGSZ1, a G(z)-selective rgs protein in brain: Structure, membrane association, regulation by Gα(z) phosphorylation, and relationship to a G(z) gtpase-activating protein subfamily

We cloned the cDNA for human RGSZ1, the major Gz-selective GTPase-activating protein (GAP) in brain (Wang, J., Tu, Y., Woodson, J., Song, X., and Ross, E. M. (1997)J. Biol. Chem. 272, 5732–5740) and a member of the RGS family of G protein GAPs. Its sequence is 83% identical to RET-RGS1 (except its N-terminal extension) and 56% identical to GAIP. Purified, recombinant RGSZ1, RET-RGS1, and GAIP each accelerated the hydrolysis of Gαz-GTP over 400-fold withK m values of ∼2 nm. RGSZ1 was 100-fold selective for Gαz over Gαi, unusually specific among RGS proteins. Other enzymological properties of RGSZ1, brain Gz GAP, and RET-RGS1 were identical; GAIP differed only in Mg2+ dependence and in its slightly lower selectivity for Gαz. RGSZ1, RET-RGS1, and GAIP thus define a subfamily of Gz GAPs within the RGS proteins. RGSZ1 has no obvious membrane-spanning region but is tightly membrane-bound in brain. Its regulatory activity in membranes depends on stable bilayer association. When co-reconstituted into phospholipid vesicles with Gz and m2 muscarinic receptors, RGSZ1 increased agonist-stimulated GTPase >15-fold with EC50<12 nm, but RGSZ1 added to the vesicle suspension was <0.1% as active. RGSZ1, RET-RGS1, and GAIP share a cysteine string sequence, perhaps targeting them to secretory vesicles and allowing them to participate in the proposed control of secretion by Gz. Phosphorylation of Gαz by protein kinase C inhibited the GAP activity of RGSZ1 and other RGS proteins, providing a mechanism for potentiation of Gz signaling by protein kinase C.