Synthesis and effect of nonhydrolyzable xanthosine triphosphate derivatives on prenylation of Rab5D136N

Identification of hydrolytically stable and selective P2Y(1) receptor agonists

P2Y nucleotide receptors (P2YRs) are attractive pharmaceutical targets. Most P2YR agonists proposed as drugs consist of a nucleotide scaffold, but their use is limited due to their chemical and enzymatic instabilities. To identify drug candidates, we developed non-hydrolyzable P2YR agonists. We synthesized ATP-beta,gamma-CH(2) analogues 2-4, and evaluated their chemical and metabolic stabilities and activities at P2Y(1,2,4,6) receptors. Analogues 2-4 exhibited t(1/2) values of 14.5-65 h in gastric juice pH. They were completely resistant to alkaline phosphatase for 30 min at 37 degrees C and slowly hydrolyzed in human blood serum (t(1/2) 12.7-71.9 h). In comparison to ATP, analogues 2-4 were barely hydrolyzed by nucleoside triphosphate diphosphohydrolases, NTPDase1,2,3,8 (< 8% hydrolysis), and nucleotide pyrophosphatases, NPP1,3 (< or = 10% hydrolysis). Analogues 2 and 4B were selective agonists of the P2Y(1)R with EC(50)s of 0.08 and 17.2 microM, respectively. These features make analogues 2 and 4B potential therapeutic agents for health disorders involving the P2Y(1)R.

Xanthine nucleotide-specific G-protein alpha-subunits: a novel approach for the analysis of G-protein-mediated signal transduction

Pro- and eukaryotic cells express multiple GTP-binding proteins that play crucial roles in signal transduction. GTP-binding proteins possess a highly conserved NKX D motif critically involved in guanine binding. In order to selectively activate a defined GTP-binding protein, base-specificity can be switched from guanine to xanthine by mutating the conserved aspartate into asparagine (D/N-mutation). This approach was very successful at elucidating the function of structurally diverse GTP-binding proteins in complex systems. However, attempts to generate functional xanthine nucleotide-specific alpha-subunits of heterotrimeric GTP-binding proteins (G-proteins) met more difficulties. Recent studies have shown that a sufficiently high GDP-affinity is critical for functional expression of xanthine nucleotide-selective G-protein mutants. Moreover, xanthosine 5'-[gamma-thio]triphosphate and xanthosine 5'-[gamma, beta-imido]triphosphate are not functionally equivalent activators of D/N-G-protein mutants. We are now in the position to exploit xanthine nucleotide-specific G-proteins to dissect signaling pathways activated by a given G-protein in complex systems.

GDP affinity and order state of the catalytic site are critical for function of xanthine nucleotide-selective Galphas proteins

Xanthine nucleotide-selective small GTP-binding proteins with an Asp/Asn mutation are valuable for the analysis of individual GTP-binding proteins in complex systems. Similar applications can be devised for heterotrimeric G-proteins. However, Asp/Asn mutants of Galpha(o), Galpha(11), and Galpha(16) were inactive. An additional Gln/Leu mutation in the catalytic site, reducing GTPase activity and increasing GDP affinity, was required to generate xanthine nucleotide-selective unspecified G-protein alpha-subunit (Galpha). Our study aim was to generate xanthine nucleotide-selective mutants of Galpha(s), the stimulatory G-protein of adenylyl cyclase. The short splice variant of Galpha(s) (Galpha(sS)) possesses higher GDP affinity than the long splice variant (Galpha(sL)). Nucleoside 5'-[gamma-thio]triphosphates (NTPgammaSs) and nucleoside 5'-[beta,gamma-imido]triphosphates effectively activated a Galpha(sS) mutant with a D280N exchange (Galpha(sS)-N280), whereas nucleotides activated a Galpha(sL) mutant with a D295N exchange (Galpha(sL)-N295) only weakly. The Gln/Leu mutation enhanced Galpha(sL)-N295 activity. NTPgammaSs activated Galpha(sS)-N280 and a Galpha(sL) mutant with a Q227L and D295N exchange (Galpha(sL)-L227/N295) with similar potencies, whereas xanthosine 5'-triphosphate and xanthosine 5'-[beta,gamma-imido]triphosphate were more potent than GTP and guanosine 5'-[beta,gamma-imido]triphosphate, respectively. Galpha(sS)-N280 interacted with the beta(2)-adrenoreceptor and exhibited high-affinity XTPase activity. Collectively, (i) Galpha(sS)-N280 is the first functional xanthine nucleotide-selective Galpha with the Asp/Asn mutation alone; (ii) sufficiently high GDP affinity is crucial for Galpha Asp/Asn mutant function; (iii) with nucleoside 5'-triphosphates and nucleoside 5'-[beta,gamma-imido]triphosphates, Galpha(s)-N280 and Galpha(sL)-L227/N295 exhibit xanthine nucleotide selectivity, whereas NTPgammaSs sterically perturb the catalytic site of Galpha and annihilate xanthine selectivity.

Synthesis of Enzymatically Stable Analogues of GDP for Binding Studies with Transducin, the G-Protein of the Visual Photoreceptor

The synthesis of five enzymatically stable analogues of guanosine diphosphate (GDP) has been carried out. The pyrophosphate moiety was mimicked in turn by the malonate, the acetophosphonate, the phosphonoacetate, the methylene-bis-phosphonate, and the imidodiphosphate groups. All the compounds were prepared via the synthesis of a transient fully protected nucleoside diphosphate analogue, and the final deprotection step was achieved by catalytic hydrogenolysis. The biological properties of the compounds have been evaluated toward transducin, the G-protein of the visual photoreceptor. Three guanosine imidodiphosphate derivatives bearing a linker at different positions on the sugar and on the base were then prepared and evaluated, giving some insight into the GDP binding site of transducin.

Evidence for a symmetrical requirement for Rab5-GTP in in vitro endosome-endosome fusion

Early endosome fusion, which has been extensively characterized using an in vitro reconstitution assay, is Rab5-dependent. To examine the requirement for Rab5 on both fusion partners, we prepared cytosol and endosomes depleted of Rab5. Unlike control cytosol, Rab5-depleted cytosol was only marginally active in the in vitro endosome fusion. However, fusion could be restored by the addition of wild-type Rab5 or Rab5 D136N, a mutant whose nucleotide specificity favors xanthine over guanine. The addition of Rab5 D136N restored fusion only in the presence of XTP. In the absence of XTP or in the presence of XDP, Rab5 D136N failed to restore fusion. When fusion was carried out with endosomal vesicles depleted of Rab GTPases (by preincubation of vesicles with GDP dissociation inhibitor), together with cytosol immunodepleted of Rab5, fusion was virtually absent. We then used immunodepleted cytosol and GDP dissociation inhibitor-treated vesicles to determine whether Rab5 is required by both fusion partners. Using separate sets of endosomal vesicles, we found that priming both sets of Rab5-depleted vesicles with Rab5 Q79L, a GTPase-defective mutant, substantially stimulated endosome fusion. Priming one set of vesicles with Rab5 Q79L and a second set of vesicles with Rab5 S34N failed to activate fusion. When both sets of Rab5-depleted vesicles were primed with Rab5 D136N supplemented with XTP, endosome fusion was stimulated, similar to that observed with Rab5 Q79L. However, when one set of vesicles was preincubated with Rab5 D136N plus XTP and the second set with Rab5 D136N and XDP, no stimulation of fusion was observed. We conclude that Rab5-GTP is required on both fusion partners for docking and fusion of early endosomes. To confirm the fusion of Rab5-GTP-positive vesicles in vivo, we expressed GFP-Rab5 Q79L in fibroblasts and observed fusion of Rab5-positive vesicles. We failed to record fusion of Rab5-positive vesicles with Rab5-negative vesicles. We conclude that Rab5-GTP is required on both sets of endosomes for fusion in vitro and in living cells.

Empty site forms of the SRP54 and SR alpha GTPases mediate targeting of ribosome-nascent chain complexes to the endoplasmic reticulum

The SRP54 and SR alpha subunits of the signal recognition particle (SRP) and the SRP receptor (SR) undergo a tightly coupled GTPase cycle that mediates the signal sequence-dependent attachment of ribosomes to the Sec61 complex. Here, we show that SRP54 and SR alpha are in the empty site conformation prior to contact between the SRP-ribosome complex and the membrane-bound SR. Cooperative binding of GTP to SRP54 and SR alpha stabilizes the SRP-SR complex and initiates signal sequence transfer from SRP54 to Sec61 alpha. The GTP-bound conformations of SR alpha and SRP54 perform distinct roles, with SR alpha performing a predominant role in complex stabilization. Hydrolysis by both SRP54 and SR alpha is a prerequisite for dissociation of the SRP-SR complex.