GTP-binding Proteins in Human Platelets

A low M(r) GTP-binding protein, Rap1, in human platelets: localization, translocation and phosphorylation by cyclic AMP-dependent protein kinase

Subcellular fractions were prepared from human platelet membranes by sucrose density gradient centrifugation and the localization of a low M(r) GTP-binding protein, rap1 protein (Rap1) was analysed by immunoblotting using a specific antibody. Rap1, which has been purified from human platelets, was found to be located in plasma membrane and alpha-granule fractions in resting platelets. Treatment of isolated alpha-granules with pronase led to proteolysis of Rap1, indicating that this protein is exposed to the cytoplasmic face of the granules. Degranulation of alpha-granules consists of translocation and subsequent fusion of the granules with the open canalicular system. Activation of this process by thrombin induced the redistribution of Rap1 on the alpha-granules to plasma membranes. On the other hand, Rap1 is known to be phosphorylated by cyclic AMP-dependent protein kinase (A-kinase) in vitro and in vivo. In intact human platelets, phosphorylation of Rap1 by A-kinase in response to prostaglandin E1 (PGE1) was observed only in Rap1 localized in plasma membranes and not on alpha-granules, although Rap1 was phosphorylated in a cell-free system when plasma membranes and alpha-granule membranes were exposed to A-kinase as substrates. These results strongly suggest that Rap1 in plasma membranes and the protein on alpha-granules are regulated by different mechanisms, and have different functions.

Intracellular calcium mobilization is triggered by clustering of membrane glycoproteins in concanavalin A-stimulated platelets

Stimulation of human platelets with concanavalin A resulted in a significant increase in the concentration of cytoplasmic free Ca2+. This effect was due to two different processes: Ca2+ mobilization from internal stores and Ca2+ influx from the extracellular medium. Kinetic analysis revealed that the release of Ca2+ from internal storage sites occurred sooner than the opening of plasma membrane Ca2+ channels. The ability of concanavalin A to induce a sustained increase in cytoplasmic Ca2+ concentration was antagonized and reversed by methyl alpha-D-mannopyranoside, demonstrating that it was promoted by the interaction of the lectin with cell surface glycoproteins. Succinyl-concanavalin A, a dimeric derivative of the lectin, that does not promote patching/capping of the receptor, was able to bind to the platelet surface, and antagonized the effects of native concanavalin A. In addition, succinyl-concanavalin A, per se, was unable to induce Ca2+ mobilization in human platelets. Therefore, the action of the native concanavalin A was mediated by receptor clustering events. Concanavalin A mobilized Ca2+ from the same internal stores from which Ca2+ was mobilized in response to strong platelet agonists, such as thrombin and arachidonic acid. However, while thrombin was ineffective in inducing Ca2+ release after stimulation of platelets with ConA, ConA was able to cause a full discharge of Ca2+ from internal stores even in platelets previously stimulated with thrombin. These results demonstrate for the first time that the clustering of specific membrane glycoproteins can trigger platelet activation. The physiological implications during platelet aggregation are discussed.

Purification and characterization of a low M(r) GTP-binding protein, c25KG, from human platelet membranes

A low M(r) GTP-binding protein with a M(r) of 26,000 has been purified from a sodium cholate extract of human platelet membranes by using an antibody raised against a synthetic peptide of c25KG, which was previously purified from human platelet cytosol (Nagata, N., et al. (1989) J. Biol. Chem. 264, 17000-17005). The M(r) of membranous c25KG (m-c25KG) was slightly higher than that from cytosolic c25KG (M(r) 25,000) and calculated to be 26,000. It was suggested that m-c25KG contains an equimolar amount of GDP. The purified protein could bind approx. 1 mol of [35S]guanosine 5'-O-(thiotriphosphate)(GTP gamma S)/mol of protein, with a Kd value of 50 nM. [35S]GTP gamma S-binding to this protein was inhibited by GTP and GDP, but not by ATP and ADP, showing that the binding is specific for guanine. In the presence of 10 mM Mg2+, the dissociation of [8,5'-3H]GDP from the m-c25KG occurred with a rate of 0.01 min-1. The rate of release of Pi from [gamma-32P]GTP-bound m-c25KG was calculated to be 0.03 min-1. These results indicate that c25KG is also present in membrane fraction of human platelet which has very similar biochemical properties in those of the cytosolic type.

Occurrence and biochemical characterization of GTP-binding proteins in Candida albicans

The activities of GTP-binding and GTPase in Candida albicans were present in the cytosol, KCl- and cholate-extractable fractions. At least two kinds of GTP-binding proteins were found in the cytosolic fraction; the major one with a molecular mass of about 30 kDa and the other about 500 kDa. The former specifically bound guanine nucleotides and was most likely to bind GDP since guanosine 5'-O-(thio) triphosphate (GTP gamma S)-binding was accelerated by addition of (NH4)2SO4. The latter showed no specificity in nucleotide binding and could also bind adenine nucleotides. The proteins were not ADP-ribosylated by either pertussis toxin or cholera toxin. These results indicate that ras-like monomeric, low molecular mass GTP-binding proteins distinct from heterotrimeric G proteins such as Gi, Go and Gs are present in C. albicans.

Platelet glycoprotein IIb-IIIa is associated with 21-kDa GTP-binding protein

Platelet membrane glycoprotein IIb-IIIa has been widely studied in the last years because of its role as an activation-dependent, adhesive protein receptor. Recently we demonstrated that occupancy of glycoprotein IIb-IIIa-receptor sites by specific ligands exerts an inhibitory effect on platelet responses induced by mild stimulation, leading us to suppose that this event may interact with activation pathways. Although the mechanisms of signal transduction in human platelets are not completely elucidated, the hypothesis that GTP-binding proteins are involved is generally accepted. Our results demonstrate that platelet ConA receptors, known to be located mainly on GP IIb-IIIa, are able to bind [35S]GTP gamma S; the GTP-binding activity is specific and is due to the association with the receptors of two G-proteins, with apparent molecular masses of 25 and 21 kDa, respectively. After the purification of GP IIb-IIIa, a glycoprotein complex electrophoretically pure was obtained that was still associated with a GTP-binding activity, migrating in SDS-polyacrylamide gel electrophoresis as a narrow band of about 21 kDa.

The ram: a novel low molecular weight GTP-binding protein cDNA from a rat megakaryocyte library

A novel low Mr GTP-binding protein cDNA was isolated from a rat megakaryocyte cDNA library with a synthetic oligonucleotide probe corresponding to an 8-amino acid sequence specific for c25KG, a GTP-binding protein previously isolated from human platelet cytosol fraction [(1989) J. Biol. Chem. 264, 17000-17005]. The cDNA has an open reading frame encoding a protein of 221 amino acids with a calculated Mr of 25068. The protein is designated as ram (ras-related gene from megakaryocyte) protein (ram p25). The amino acid sequence deduced from the ram cDNA contains the consensus sequences for GTP-binding and GTPase domains. ram p25 shares about 23%, 39% and 80% amino acid homology with the H-ras, smg25A and c25KG proteins, respectively. The 3.5-kb ram mRNA was detected abundantly in spleen cells.