Gasp: not just another oncogene

Structure at 1.65 A of RhoA and its GTPase-activating protein in complex with a transition-state analogue

Small G proteins of the Rho family, which includes Rho, Rac and Cdc42Hs, regulate phosphorylation pathways that control a range of biological functions including cytoskeleton formation and cell proliferation. They operate as molecular switches, cycling between the biologically active GTP-bound form and the inactive GDP-bound state. Their rate of hydrolysis of GTP to GDP by virtue of their intrinsic GTPase activity is slow, but can be accelerated by up to 10(5)-fold through interaction with rhoGAP, a GTPase-activating protein that stimulates Rho-family proteins. As such, rhoGAP plays a crucial role in regulating Rho-mediated signalling pathways. Here we report the crystal structure of RhoA and rhoGAP complexed with the transition-state analogue GDP.AlF4- at 1.65 A resolution. There is a rotation of 20 degrees between the Rho and rhoGAP proteins in this complex when compared with the ground-state complex Cdc42Hs.GMPPNP/rhoGAP, in which Cdc42Hs is bound to the non-hydrolysable GTP analogue GMPPNP. Consequently, in the transition state complex but not in the ground state, the rhoGAP domain contributes a residue, Arg85(GAP) directly into the active site of the G protein. We propose that this residue acts to stabilize the transition state of the GTPase reaction. RhoGAP also appears to function by stabilizing several regions of RhoA that are important in signalling the hydrolysis of GTP.

Ras-catalyzed hydrolysis of GTP: a new perspective from model studies

Despite the biological and medical importance of signal transduction via Ras proteins and despite considerable kinetic and structural studies of wild-type and mutant Ras proteins, the mechanism of Ras-catalyzed GTP hydrolysis remains controversial. We take a different approach to this problem: the uncatalyzed hydrolysis of GTP is analyzed, and the understanding derived is applied to the Ras-catalyzed reaction. Evaluation of previous mechanistic proposals from this chemical perspective suggests that proton abstraction from the attacking water by a general base and stabilization of charge development on the gamma-phosphoryl oxygen atoms would not be catalytic. Rather, this analysis focuses attention on the GDP leaving group, including the beta-gamma bridge oxygen of GTP, the atom that undergoes the largest change in charge in going from the ground state to the transition state. This leads to a new catalytic proposal in which a hydrogen bond from the backbone amide of Gly-13 to this bridge oxygen is strengthened in the transition state relative to the ground state, within an active site that provides a template complementary to the transition state. Strengthened transition state interactions of the active site lysine, Lys-16, with the beta-nonbridging phosphoryl oxygens and a network of interactions that positions the nucleophilic water molecule and gamma-phosphoryl group with respect to one another may also contribute to catalysis. It is speculated that a significant fraction of the GAP-activated GTPase activity of Ras arises from an additional interaction of the beta-gamma bridge oxygen with an Arg side chain that is provided in trans by GAP. The conclusions for Ras and related G proteins are expected to apply more widely to other enzymes that catalyze phosphoryl (-PO(3)2-) transfer, including kinases and phosphatases.

Molecular pathogenesis of pituitary tumours

Human pituitary tumours account for 10% of intracranial neoplasms. These tumours are usually sporadic and benign; malignant change and metastasis are extremely rare events. Autosomal dominant inheritance of MEN 1 accounts for a minority of pituitary tumours. Pituitary tumours have been found to be monoclonal in several studies. This would suggest that an intrinsic genetic pituitary defect is pivotal in the pathogenesis of these tumours. However, this concept does not exclude a role for the hypothalamus in the genesis of pituitary tumours; the trophic function of several hypothalamic peptides could promote initiation of the genetic event or facilitate a sequence of events leading to clonal expansion of the transformed cell. There has been modest progress made in the elucidation of the intrinsic genetic pituitary cell abnormalities that underlie pituitary tumorigenesis. A mutant alpha subunit of the Gs gene, designated gsp, which results in constitutive activation of adenylyl cylcase has been described in a subset of GH cell adenomas. Loss of genetic material on chromosome 11q13, the locus of the MEN 1 gene, is found in under 20% of pituitary adenomas, suggesting that inactivation of a tumour suppressor gene at this locus may be significant in the tumorigenic process. H-ras point mutations have been described in distant metastatic pituitary tumour secondaries. The genetic abnormalities described occur in only a small subset of pituitary tumours, indicating that the more significant tumour promoting genes are still to be discovered.

Suppression of Ras-induced transformation of NIH 3T3 cells by activated G alpha s

Conversion of external signals into proliferative responses may be mediated by interactions between signaling pathways that control cell proliferation. Interactions between G alpha s, the alpha subunit of the heterotrimeric guanine nucleotide binding protein that stimulates adenylyl cyclase, and Ras, an important element in growth factor signaling, were studied. Expression of activated G alpha s in NIH 3T3 cells increased intracellular concentrations of adenosine 3',5'-monophosphate (cAMP) and inhibited H-Ras-stimulated DNA synthesis and mitogen-activated protein kinase activity. Activated G alpha s and 8-Br-cAMP suppressed H-Ras-induced transformation of NIH 3T3 cells. Apparently, G alpha s inhibits proliferative signals from Ras by stimulating cAMP production and activating protein kinase A.

Clinical and biochemical characteristics of acromegalic patients harboring gsp-positive and gsp-negative pituitary tumors

A subgroup of growth hormone (GH)-secreting pituitary tumors carries somatic mutations within the gene coding for the alpha subunit of the stimulatory heterotrimeric guanosine 5'-triphosphate-binding protein, Gs alpha. These so-called gsp mutations result in constitutively activated Gs alpha and the signal transduction cascade downstream of it, with eventual markedly and continuously elevated cyclic adenosine monophosphate levels as a result of constitutive adenylyl cyclase activity. It is this elevation of intracellular cyclic adenosine monophosphate that is thought to be the cause of excessive GH secretion and somatotroph proliferation. We examined the clinical and biochemical characteristics of acromegalics harboring gsp-positive and gsp-negative pituitary tumors. Of 19 tumors studied, 8 (42%) were gsp positive. There was a slight tendency for basal GH levels in serum to be lower and to be further reduced by an oral glucose tolerance test in gsp-positive patients. However, there was no difference between the two groups in terms of clinical features, tumor size, mitotic activity (as assessed by cytosolic deoxyribonucleic acid polymerase and KI-67 staining), and in vitro GH response to GH releasing factor. We conclude that there is, in general, little difference in the clinical and biochemical characteristics between gsp-positive and gsp-negative human pituitary GH-secreting tumors.

Interactions of the ras-like protein p25rab3A with Mg2+ and guanine nucleotides

The rab3A gene product is a 25 kDa guanine-nucleotide-binding protein which is expressed at high levels in neural tissue and has about 30% sequence similarity to ras. Purified p25rab3A has been used as substrate to examine its kinetics of nucleotide binding and hydrolysis, and to study the effects of Mg2+ on these processes. p25rab3A binds GDP and GTP similarly well, with nanomolar affinity. Mg2+ increases the affinity between p25rab3A and guanine nucleotides by 3- and 7-fold for GTP and GDP respectively, primarily by drastically decreasing the nucleotide off-rates. The Mg2+ binding affinity to p25rab3A. [alpha 32P]GDP was determined to be about 4 microns using entrapment of [alpha-32P]GDP as a measure of Mg2+ binding. At a Mg2+ concentration of 11 mM. GTPase activity was rate-limited by the GDP off-rate. Surprisingly, at a Mg2+ concentration of 80 nM. GTPase activity was comparable with that in the presence of excess Mg2+. In this case, kcat. was rate-limiting. At Mg2+ concentrations below 10 nM there was no detectable GTPase activity, indicating that Mg2+ is required for the GTPase activity of p25rab3A.

Dopamine, the dopamine D2 receptor and pituitary tumours

Dopamine plays an important role in the hypothalamic-pituitary axis where its major effects are to inhibit pituitary hormone secretion and cell division. Chronic dopamine deficiency has been postulated as a cause of pituitary tumour formation and several lines of evidence exist to suggest that a functional deficiency may develop as a result of defective dopamine receptor action. The available data suggest that a number of sites in the dopamine-D2 receptor-second messenger pathways may be implicated. These abnormalities are reflected in the variety of responses to dopamine and its agonists which have been observed in pituitary tumours both in the clinical situation and in cultured cells in vitro. Whilst it seems likely that the primary defect in pituitary tumour formation lies within the pituitary itself, the role of hypothalamic factors in facilitating tumour growth remains to be explored. Further studies of the dopamine receptor and its function will be of value not only in pathophysiological studies of human pituitary adenomas, but also in the development of new pharmacological agents to treat patients with these tumours.

Molecular cloning and sequence determination of four different cDNA species coding for alpha-subunits of G proteins from Xenopus laevis oocytes

A cDNA library prepared from Xenopus laevis oocytes in lambda gt10 was screened with a mixture of three oligonucleotide probes designed to detect sequences found in different mammalian genes coding for alpha-subunits of G-proteins. In addition to a clone coding for a G alpha o-type subunit previously reported [(1989) FEBS Lett. 244, 188-192] four additional clones have been found coding for different G alpha protein subunits. By comparison with mammalian alpha-subunits, these oocyte cDNAs correspond to two closely related G alpha s-1a, to a G alpha i-1 and to a G alpha i-3 species. The derived amino acid sequences showed that both G alpha s species contain 379 residues, corresponding to the short species without the serine residue and with a calculated Mr of 42720. The G alpha i-1 gene encodes a 354 amino acid protein with an Mr of 39,000 and the G alpha i-3 encodes an incomplete open reading frame of 345 residues, lacking the first 9 amino acid residues at the NH2 terminus. All these G alpha-subunits showed high identity with their respective mammalian counterparts (75-80%), indicating a great degree of conservation through the evolution and the important cellular regulatory function that they play.