High-throughput assay for profiling the substrate specificity of Rab GTPase-activating proteins

Systems-level effects of allosteric perturbations to a model molecular switch

Molecular switch proteins whose cycling between states is controlled by opposing regulators^1,2 are central to biological signal transduction. As switch proteins function within highly connected interaction networks³, the fundamental question arises of how functional specificity is achieved when different processes share common regulators. Here we show that functional specificity of the small GTPase switch protein Gsp1 in Saccharomyces cerevisiae (the homologue of the human protein RAN)⁴ is linked to differential sensitivity of biological processes to different kinetics of the Gsp1 (RAN) switch cycle. We make 55 targeted point mutations to individual protein interaction interfaces of Gsp1 (RAN) and show through quantitative genetic⁵ and physical interaction mapping that Gsp1 (RAN) interface perturbations have widespread cellular consequences. Contrary to expectation, the cellular effects of the interface mutations group by their biophysical effects on kinetic parameters of the GTPase switch cycle and not by the targeted interfaces. Instead, we show that interface mutations allosterically tune the GTPase cycle kinetics. These results suggest a model in which protein partner binding, or post-translational modifications at distal sites, could act as allosteric regulators of GTPase switching. Similar mechanisms may underlie regulation by other GTPases, and other biological switches. Furthermore, our integrative platform to determine the quantitative consequences of molecular perturbations may help to explain the effects of disease mutations that target central molecular switches.

Newer Methods Drive Recent Insights into Rab GTPase Biology: An Overview

The conserved Ypt/Rab GTPases regulate all major intracellular protein traffic pathways, including secretion, endocytosis and autophagy. These GTPases undergo distinct changes in conformation between their GTP- and GDP-bound forms and cycle between the cytoplasm and membranes with the aid of their upstream regulators. When activated on the membrane in the GTP-bound form, they recruit their downstream effectors, which include components of vesicular transport. Progress in the past 5 years regarding mechanisms of Rab action, functions, and the effects of disruption of these functions on the well-being of cells and organisms has been propelled by advances in methodologies in molecular and cellular biology. Here, we highlight methods used recently to analyze regulation, localization, interactions, and function of Rab GTPases and their roles in human disease. We discuss contributions of these methods to new insights into Rabs, as well as their future use in addressing open questions in the field of Rab biology.

Shigella escapes lysosomal degradation through inactivation of Rab31 by IpaH4.5

Introduction. Shigella flexneri is an intracellular bacterial pathogen that utilizes a type III secretion apparatus to inject effector proteins into host cells.Hypothesis/Gap Statement. The T3SS effector IpaH4.5 is important for the virulence of Shigella.Aim. This study aimed to elucidate the molecular mechanism and host target of the IpaH4.5 as well as its roles in S. flexneri infection.Methodology. The GAP assay was used to identify substrate Rab GTPases of IpaH4.5. A coimmunoprecipitation assay was applied to identify the interaction of Rab GTPases with IpaH4.5. A confocal microscopy analysis was used to assess the effects of IpaH4.5 on mannose 6-phosphate receptor (MPR) trafficking. To identify the effects of IpaH4.5 GAP activity on the activity of lysosomal cathepsin B, the Magic Red-RR assay was used. Finally, the intracellular persistence assay was used to identify IpaH4.5 GAP activity in S. flexneri intracellular growth.Results. We found that the effector IpaH4.5 disrupts MPR trafficking and lysosomal function, thereby counteracting host lysosomal degradation. IpaH4.5 harbours TBC-like dual-finger motifs and exhibits potent RabGAP activities towards Rab31. IpaH4.5 disrupts the transport of the cation-dependent mannose 6-phosphate receptor (CD-MPR) from the Golgi to the endosome by targeting Rab31, thereby attenuating lysosomal function. As a result, the intracellular persistence of S. flexneri requires IpaH4.5 TBC-like GAP activity to mediate bacterial escape from host lysosome-mediated elimination.Conclusion. We identified an unknown function of IpaH4.5 and its potential role in S. flexneri infection.

High-Throughput Assay for Profiling the Substrate Specificity of Rab GTPase-Activating Proteins

Measurement of intrinsic as well as GTPase-activating Protein (GAP) catalyzed GTP hydrolysis is central to understanding the molecular mechanism and function of GTPases in diverse cellular processes. For the Rab GTPase family, which comprises at least 60 distinct proteins in humans, putative GAPs have been identified from both eukaryotic organisms and pathogenic bacteria. A major obstacle has involved identification of target substrates and determination of the specificity for the Rab family. Here, we describe a sensitive, high-throughput method to quantitatively profile GAP activity for Rab GTPases in microplate format based on detection of inorganic phosphate released after GTP hydrolysis. The method takes advantage of a well-characterized fluorescent phosphate sensor, requires relatively low protein concentrations, and can, in principle, be applied to any GAP-GTPase system.

Developing Colorimetric and Luminescence-Based High-Throughput Screening Platforms for Monitoring the GTPase Activity of Ferrous Iron Transport Protein B (FeoB)

Iron is an essential requirement for the survival and virulence for bacteria. The bacterial ferrous iron transporter protein B (FeoB) functions as a major iron transporter in prokaryotes and has an N-terminal domain (NFeoB) with homology to eukaryotic G-proteins. Its GTPase activity is required for ferrous iron uptake, making it a potential target for antivirulence therapies. Here, two assay strategies relying on different spectroscopic readouts are described to monitor NFeoB GTPase activity. The first one is the colorimetric-based platform that utilizes a malachite green reagent to monitor phosphate production from GTP hydrolysis. The absorbance change directly relates to the GTPase activity of NFeoB. The assay was further improved by the addition of Tween-20 and miniaturized in a 384-well plate format with a 10 µL assay volume. The second format is a luminescence-based platform, measuring the GTP depletion by using a modified GTPase-Glo assay from Promega. In this platform, the luminescence signal correlates to the amount of GTP remaining, allowing for the direct calculation of GTP hydrolysis by NFeoB. The colorimetric platform was tested in a high-throughput manner against a custom-assembled library of a~2000 small molecules and was found to be simple, cost-effective, and robust. Additionally, the luminescence-based platform demonstrated its capability as an orthogonal assay to monitor GTPase activity, providing a valid and convenient method to filter false hits. These two assay platforms are proven to offset the limitations of each platform while enhancing overall quality and success rates.

TBC1D5 controls the GTPase cycle of Rab7b

Rab GTPases are key regulators of intracellular trafficking, and cycle between a GTP-bound active state and a GDP-bound inactive state. This cycle is regulated by guanine-nucleotide exchange factors (GEFs) and GTPase-activating proteins (GAPs). Several efforts have been made in connecting the correct GEFs and GAPs to their specific Rab. Here, we aimed to identify GAPs for Rab7b, the small GTPase involved in transport from late endosomes to the trans-Golgi. An siRNA screen targeting proteins containing TBC domains critical for Rab GAPs was performed and coupled to a phenotypic read-out that visualized the distribution of Rab7b. Silencing of TBC1D5 provided the strongest phenotype and this protein was subsequently validated in various in vitro and cell-based assays. TBC1D5 localizes to Rab7b-positive vesicles, interacts with Rab7b and has GAP activity towards Rab7b in vitro, which is further increased by retromer proteins. Similarly to the constitutively active mutant of Rab7b, inactivation of TBC1D5 also reduces the number of CI-MPR- and sortilin-positive vesicles. Together, the results show that TBC1D5 is a GAP for Rab7b in the control of endosomal transport to the trans-Golgi.This article has an associated First Person interview with the first author of the paper.

A RasGAP, DAB2IP, regulates lipid droplet homeostasis by serving as GAP toward RAB40C

Lipid droplet (LD) homeostasis involves activities of various RAB small GTPases. Recently, we found RAB40C was one of the RAB proteins regulating LD homeostasis. RAB40C contains a unique SOCS domain that is required for clustering of LDs. However, its precise functional role in LD homeostasis and mechanism of regulation remain largely unknown. In this study, we observed over-accumulation of LDs in cells with RAB40C deleted by Crispr-Cas9 editing. RAB40C appeared to reduce LD accumulation after long term incubation of cells with oleic acid (24 hours). Unexpectedly, we found that Ras GTPase activating protein (GAP), DAB2IP, bound to RAB40C mainly via its GAP domain and could serve as RAB40C GAP. Studies involving overexpression of DAB2IP and its GAP defective mutant and siRNA depletion of DAB2IP all confirmed that DAB2IP negatively regulated the effect of RAB40C on LD homeostasis. These results provide a novel perspective on the regulation of RAB40C and implicate various signalling pathways regulated by DAB2IP, which may play a role in LD homeostasis via RAB40C.

Invited review: Small GTPases and their GAPs

Widespread utilization of small GTPases as major regulatory hubs in many different biological systems derives from a conserved conformational switch mechanism that facilitates cycling between GTP-bound active and GDP-bound inactive states under control of guanine nucleotide exchange factors (GEFs) and GTPase activating proteins (GAPs), which accelerate slow intrinsic rates of activation by nucleotide exchange and deactivation by GTP hydrolysis, respectively. Here we review developments leading to current understanding of intrinsic and GAP catalyzed GTP hydrolytic reactions in small GTPases from structural, molecular and chemical mechanistic perspectives. Despite the apparent simplicity of the GTPase cycle, the structural bases underlying the hallmark hydrolytic reaction and catalytic acceleration by GAPs are considerably more diverse than originally anticipated. Even the most fundamental aspects of the reaction mechanism have been challenging to decipher. Through a combination of experimental and in silico approaches, the outlines of a consensus view have begun to emerge for the best studied paradigms. Nevertheless, recent observations indicate that there is still much to be learned. © 2016 Wiley Periodicals, Inc. Biopolymers 105: 431-448, 2016.