Role of Rab9 GTPase in facilitating receptor recruitment by TIP47

Mannose 6-phosphate receptors (MPRs) deliver lysosomal hydrolases from the Golgi to endosomes and then return to the Golgi complex. TIP47 recognizes the cytoplasmic domains of MPRs and is required for endosome-to-Golgi transport. Here we show that TIP47 also bound directly to the Rab9 guanosine triphosphatase (GTPase) in its active, GTP-bound conformation. Moreover, Rab9 increased the affinity of TIP47 for its cargo. A functional Rab9 binding site was required for TIP47 stimulation of MPR transport in vivo. Thus, a cytosolic cargo selection device may be selectively recruited onto a specific organelle, and vesicle budding might be coupled to the presence of an active Rab GTPase.     

Rab1 recruitment of p115 into a cis-SNARE complex: programming budding COPII vesicles for fusion

The guanosine triphosphatase Rab1 regulates the transport of newly synthesized proteins from the endoplasmic reticulum to the Golgi apparatus through interaction with effector molecules, but the molecular mechanisms by which this occurs are unknown. Here, the tethering factor p115 was shown to be a Rab1 effector that binds directly to activated Rab1. Rab1 recruited p115 to coat protein complex II (COPII) vesicles during budding from the endoplasmic reticulum, where it interacted with a select set of COPII vesicle-associated SNAREs (soluble N-ethylmaleimide-sensitive factor attachment protein receptors) to form a cis-SNARE complex that promotes targeting to the Golgi apparatus. We propose that Rab1-regulated assembly of functional effector-SNARE complexes defines a conserved molecular mechanism to coordinate recognition between subcellular compartments.     

A bifunctional bacterial protein links GDI displacement to Rab1 activation

Rab guanosine triphosphatases (GTPases) regulate vesicle trafficking in eukaryotic cells by reversibly associating with lipid membranes. Inactive Rab GTPases are maintained in the cytosol by binding to GDP-dissociation inhibitor (GDI). It is believed that specialized proteins are required to displace GDI from Rab GTPases before Rab activation by guanosine diphosphate-guanosine 5'-triphosphate (GDP-GTP) exchange factors (GEFs). Here, we found that SidM from Legionella pneumophila could act as both GEF and GDI-displacement factor (GDF) for Rab1. Rab1 released from GDI was inserted into liposomal membranes and was used as a substrate for SidM-mediated nucleotide exchange. During host cell infection, recruitment of Rab1 to Legionella-containing vacuoles depended on the GDF activity of SidM. Thus, GDF and GEF activity can be promoted by a single protein, and GDF activity can coordinate Rab1 recruitment from the GDI-bound pool.     

The mechanism for activation of GTP hydrolysis on the ribosome

Protein synthesis requires several guanosine triphosphatase (GTPase) factors, including elongation factor Tu (EF-Tu), which delivers aminoacyl-transfer RNAs (tRNAs) to the ribosome. To understand how the ribosome triggers GTP hydrolysis in translational GTPases, we have determined the crystal structure of EF-Tu and aminoacyl-tRNA bound to the ribosome with a GTP analog, to 3.2 angstrom resolution. EF-Tu is in its active conformation, the switch I loop is ordered, and the catalytic histidine is coordinating the nucleophilic water in position for inline attack on the γ-phosphate of GTP. This activated conformation is due to a critical and conserved interaction of the histidine with A2662 of the sarcin-ricin loop of the 23S ribosomal RNA. The structure suggests a universal mechanism for GTPase activation and hydrolysis in translational GTPases on the ribosome.     

Regulatory role for GTP-binding proteins in endocytosis

Guanosine 5'-triphosphate (GTP)-binding proteins have been implicated in the transport of newly synthesized proteins along the secretory pathway of yeast and mammalian cells. Early vesicle fusion events that follow receptor-mediated endocytosis as measured by three in vitro assays were blocked by guanosine 5'-O-(3-thiotriphosphate) and aluminum fluoride. The effect was specific for guanosine nucleotides and depended on the presence of cytosolic factors. Thus, GTP-binding proteins may also have a role in the transport of molecules along the endocytic pathway.     

De-AMPylation of the small GTPase Rab1 by the pathogen Legionella pneumophila

The bacterial pathogen Legionella pneumophila exploits host cell vesicle transport by transiently manipulating the activity of the small guanosine triphosphatase (GTPase) Rab1. The effector protein SidM recruits Rab1 to the Legionella-containing vacuole (LCV), where it activates Rab1 and then AMPylates it by covalently adding adenosine monophosphate (AMP). L. pneumophila GTPase-activating protein LepB inactivates Rab1 before its removal from LCVs. Because LepB cannot bind AMPylated Rab1, the molecular events leading to Rab1 inactivation are unknown. We found that the effector protein SidD from L. pneumophila catalyzed AMP release from Rab1, generating de-AMPylated Rab1 accessible for inactivation by LepB. L. pneumophila mutants lacking SidD were defective for Rab1 removal from LCVs, identifying SidD as the missing link connecting the processes of early Rab1 accumulation and subsequent Rab1 removal during infection.     

Message transmission: receptor controlled adenylate cyclase system

The adenylate cyclase system is composed of an activating hormone or neurotransmitter (H), its receptor (R), the guanosine triphosphate (GTP) binding protein (Gs), and the catalytic unit (C). The activation of the receptor R involves a transient change in conformation, from a loose binding of the neurotransmitter H to an extremely tight interaction, termed locking. The system is regulated in the activation steps and also by three deactivation processes. A guanosine triphosphatase activity is built into the Gs protein so that the active GsGTP has only a limited lifetime during which it is able to activate C. In addition, the continued occupation of R by H causes desensitization of R. Finally, there are inhibitory receptors, such as alpha-adrenergic and opiate receptors, which inhibit the adenylate cyclase by way of a specific GTP binding protein (Gi). Yet to be determined are the conformational transformations of pure R on binding of an agonist or a partial agonist; the genes that code for the many different receptors that activate the adenylate cyclase, and the possibility that the G components interact with systems in the cell other than the adenylate cyclase.     

植物GTP结合蛋白的研究进展

GTP结合蛋白(G蛋白)广泛存在于生物界,在细胞信息传导中起着极为重要的开关作用,它可以调控真核生物中高度保守的跨膜信号传导通路。G蛋白可以划分为异源三聚体G蛋白和低分子量G蛋白(Ran)两种类型。Ran是一种大量分布于细胞核内的小分子的GTP酶,在核质运输以及微管蛋白成核过程中具有十分重要的作用。本文主要对G蛋白以及低分子量G蛋白Ran的研究进展做一综述。     

Mediation of the attachment or fusion step in vesicular transport by the GTP-binding Ypt1 protein

The function of the guanosine triphosphate (GTP)-binding protein Ypt1 in regulating vesicular traffic was studied in a cell-free system that reconstitutes transport from the endoplasmic reticulum to the Golgi. Blocking the Ypt1 protein activity resulted in accumulation of vesicles that act as an intermediate passing between the two compartments. The Ypt1 protein was found on the outer side of these vesicles. The transport process is completed by fusion of these vesicles with the acceptor compartment, and Ypt1 protein activity was needed for this step. Thus, a specific GTP-binding protein is required for either attachment or fusion (or both) of secretory vesicles with the acceptor compartment during protein secretion.     

Structure of Rab GDP-dissociation inhibitor in complex with prenylated YPT1 GTPase

Rab/Ypt guanosine triphosphatases (GTPases) represent a family of key membrane traffic regulators in eukaryotic cells whose function is governed by the guanosine diphosphate (GDP) dissociation inhibitor (RabGDI). Using a combination of chemical synthesis and protein engineering, we generated and crystallized the monoprenylated Ypt1:RabGDI complex. The structure of the complex was solved to 1.5 angstrom resolution and provides a structural basis for the ability of RabGDI to inhibit the release of nucleotide by Rab proteins. Isoprenoid binding requires a conformational change that opens a cavity in the hydrophobic core of its domain II. Analysis of the structure provides a molecular basis for understanding a RabGDI mutant that causes mental retardation in humans.     

条斑紫菜Rab1基因的克隆与生物信息学分析

Rab蛋白是小分子量GTP结合蛋白家族中的成员之一,它与植物抗逆性密切相关。以拟南芥Rab基因序列为探针,通过筛选条斑紫菜EST数据库中的同源序列,拼接出了其Rab1基因的含有完整开放阅读框(ORF)的cD-NA序列。根据拼接的序列设计引物,利用RT-PCR成功克隆了条斑紫菜Rab1基因的cDNA。T-A克隆后测序结果显示其cDNA含有长615bp的完整ORF,编码产物含203个氨基酸残基,分子量为22.5kDa。条斑紫菜Rab1与拟南芥Rab1聚为一类,它与多种生物的Rab1具有较高的序列一致性。     

Protrudin induces neurite formation by directional membrane trafficking

Guanosine triphosphatases of the Rab family are key regulators of membrane trafficking, with Rab11 playing a specific role in membrane recycling. We identified a mammalian protein, protrudin, that promoted neurite formation through interaction with the guanosine diphosphate (GDP)-bound form of Rab11. Phosphorylation of protrudin by extracellular signal-regulated kinase (ERK) in response to nerve growth factor promoted protrudin association with Rab11-GDP. Down-regulation of protrudin by RNA interference induced membrane extension in all directions and inhibited neurite formation. Thus, protrudin regulates Rab11-dependent membrane recycling to promote the directional membrane trafficking required for neurite formation.     

The cellular functions of small GTP-binding proteins

A substantial number of novel guanine nucleotide binding regulatory proteins have been identified over the last few years but the function of many of them is largely unknown. This article will discuss a particular family of these proteins, structurally related to the Ras oncoprotein. Approximately 30 Ras-related small guanosine triphosphate (GTP)-binding proteins are known, and from yeast to man they appear to be involved in controlling a diverse set of essential cellular functions including growth, differentiation, cytoskeletal organization, and intracellular vesicle transport and secretion.     

A 32-kD GTP-binding protein associated with the CD4-p56lck and CD8-p56lck T cell receptor complexes

The guanosine triphosphate (GTP)-binding proteins include signal-transducing heterotrimeric G proteins (for example, Gs, Gi), smaller GTP-binding proteins that function in protein sorting, and the oncogenic protein p21ras. The T cell receptor complexes CD4-p56lck and CD8-p56lck were found to include a 32- to 33-kilodalton phosphoprotein (p32) that was recognized by an antiserum to a consensus GTP-binding region in G proteins. Immunoprecipitated CD4 and CD8 complexes bound GTP and hydrolyzed it to guanosine diphosphate (GDP). The p32 protein was covalently linked to [alpha-32P]GTP by ultraviolet photoaffinity labeling. These results demonstrate an interaction between T cell receptor complexes and an intracellular GTP-binding protein.     

A cytoplasmic protein stimulates normal N-ras p21 GTPase, but does not affect oncogenic mutants

The role of guanine nucleotides in ras p21 function was determined by using the ability of p21 protein to induce maturation of Xenopus oocytes as a quantitative assay for biological activity. Two oncogenic mutant human N-ras p21 proteins, Asp12 and Val12, actively induced maturation, whereas normal Gly12 p21 was relatively inactive in this assay. Both mutant proteins were found to be associated with guanosine triphosphate (GTP) in vivo. In contrast, Gly12 p21 was predominantly guanosine diphosphate (GDP)-bound because of a dramatic stimulation of Gly12 p21-associated guanosine triphosphatase (GTPase) activity. A cytoplasmic protein was shown to be responsible for this increase in activity. This protein stimulated GTP hydrolysis by purified Gly12 p21 more than 200-fold in vitro, but had no effect on Asp12 or Val12 mutants. A similar factor could be detected in extracts from mammalian cells. It thus appears that, in Xenopus oocytes, this protein maintains normal p21 in a biologically inactive, GDP-bound state through its effect on GTPase activity. Furthermore, it appears that the major effect of position 12 mutations is to prevent this protein from stimulating p21 GTPase activity, thereby allowing these mutants to remain in the active GTP-bound state.     

Activation of muscarinic potassium currents by ATP gamma S in atrial cells

Intracellular perfusion of atrial myocytes with adenosine 5'-(gamma-thio) triphosphate (ATP gamma S), an ATP analog, elicits a progressive increase of the muscarinic potassium channel current, IK(M), in the absence of agonists. In this respect, ATP gamma S mimics the actions of guanosine triphosphate (GTP) analogs, which produce direct, persistent activation of the guanyl nucleotide-binding (G) protein controlling the K+(M) channel. The effect of ATP gamma S on IK(M), however, differs from that produced by GTP analogs in two aspects: it requires relatively large ATP gamma S concentrations, and it appears after a considerable delay, suggesting a rate-limiting step not present in similar experiments performed with guanosine 5'-(gamma-thio) triphosphate (GTP gamma S). Incubation of atrial homogenates with [35S]ATP gamma S leads to formation of significant amounts of [35S]GTP gamma S, suggesting that activation of IK(M) by ATP gamma S arises indirectly through its conversion into GTP gamma S by cellular enzymes. ATP gamma S is often used to demonstrate the involvement of protein phosphorylation in the control of various cellular processes. The finding that cytosolic application of ATP gamma S can also lead to G-protein activation implies that experiments with ATP gamma S must be interpreted with caution.     

A GDP/GTP exchange factor involved in linking a spatial landmark to cell polarity

In both animal and yeast cells, signaling pathways involving small guanosine triphosphatases (GTPases) regulate polarized organization of the actin cytoskeleton. In the budding yeast Saccharomyces cerevisiae, the Ras-like GTPase Bud1/Rsr1 and its guanosine 5'-diphosphate (GDP)/guanosine 5'-triphosphate (GTP) exchange factor Bud5 are involved in the selection of a specific site for growth, thus determining cell polarity. We found that Bud5 is localized at the cell division site and the presumptive bud site. Its localization is dependent on potential cellular landmarks, such as Bud3 and Axl2/Bud10 in haploid cells and Bud8 and Bud9 in diploid cells. Bud5 also physically interacts with Axl2/Bud10, a transmembrane glycoprotein, suggesting that a receptor-like transmembrane protein recruits a GDP/GTP exchange factor to connect an intrinsic spatial signal to oriented cell growth.     

Comment on "The mechanism for activation of GTP hydrolysis on the ribosome"

Voorhees et al. (Reports, 5 November 2010, p. 835) determined the structure of elongation factor Tu (EF-Tu) and aminoacyl-transfer RNA bound to the ribosome with a guanosine triphosphate (GTP) analog. However, their identification of histidine-84 of EF-Tu as deprotonating the catalytic water molecule is problematic in relation to their atomic structure; the terminal phosphate of GTP is more likely to be the proper proton acceptor.     

Detection of GTP-tubulin conformation in vivo reveals a role for GTP remnants in microtubule rescues

Microtubules display dynamic instability, with alternating phases of growth and shrinkage separated by catastrophe and rescue events. The guanosine triphosphate (GTP) cap at the growing end of microtubules, whose presence is essential to prevent microtubule catastrophes in vitro, has been difficult to observe in vivo. We selected a recombinant antibody that specifically recognizes GTP-bound tubulin in microtubules and found that GTP-tubulin was indeed present at the plus end of growing microtubules. Unexpectedly, GTP-tubulin remnants were also present in older parts of microtubules, which suggests that GTP hydrolysis is sometimes incomplete during polymerization. Observations in living cells suggested that these GTP remnants may be responsible for the rescue events in which microtubules recover from catastrophe.     

Nanomechanical basis of selective gating by the nuclear pore complex

The nuclear pore complex regulates cargo transport between the cytoplasm and the nucleus. We set out to correlate the governing biochemical interactions to the nanoscopic responses of the phenylalanineglycine (FG)-rich nucleoporin domains, which are involved in attenuating or promoting cargo translocation. We found that binding interactions with the transport receptor karyopherin-beta1 caused the FG domains of the human nucleoporin Nup153 to collapse into compact molecular conformations. This effect was reversed by the action of Ran guanosine triphosphate, which returned the FG domains into a polymer brush-like, entropic barrier conformation. Similar effects were observed in Xenopus oocyte nuclei in situ. Thus, the reversible collapse of the FG domains may play an important role in regulating nucleocytoplasmic transport.