Guanine nucleotide-binding proteins regulate a variety of processes, including sensual perception, protein synthesis, various transport processes, and cell growth and differentiation. They act as molecular switches and timers that cycle between inactive guanosine diphosphate (GDP)-bound and active guanosine triphosphate (GTP)-bound states. Recent structural studies show that the switch apparatus itself is a conserved fundamental module but that its regulators and effectors are quite diverse in their structures and modes of interaction. Here we will try to define some underlying principles.

https://science.sciencemag.org/content/sci/294/5545/1299.full.pdf

The guanine nucleotide-binding switch in three dimensions.

Extraction and Solubilization of Proteins for Proteomic Studies Richard M. Leimgruber Preparation of Bacterial Samples for 2-D PAGE Brian Berg Vandahl, Gunna Christiansen, and Svend Birkelund Preparation of Yeast Samples for 2-D PAGE Joakim Norbeck Preparation of Mammalian Tissue Samples for Two-Dimensional Electrophoresis Frank A. Witzmann Differential Detergent Fractionation of Eukaryotic Cells Melinda L. Ramsby and Gregory S. Makowski Serum or Plasma Sample Preparation for Two-Dimensional Gel Electrophoresis Anthony G. Sullivan, Stephen Russell, Henry Brzeski, Richard I. Somiari, and Craig D. Shriver Preparation of Plant Protein Samples for 2-D PAGE David W. M. Leung Laser-Assisted Microdissection in Proteomic Analyses Darrell L. Ellsworth, Stephen Russell, Brenda Deyarmin, Anthony G. Sullivan, Henry Brzeski, Richard I. Somiari, and Craig D. Shriver Purification of Cellular and Organelle Populations by Fluorescence-Activated Cell Sorting for Proteome Analysis William L. Godfrey, Colette J. Rudd, Sujata Iyer, and Diether Recktenwald Purification of Nucleoli From Lymphoma Cells and Solubilization of Nucleolar Proteins for 2-DE Separation Regis Dieckmann, Yohann Coute, Denis Hochstrasser, Jean-Jacques Diaz, and Jean-Charles Sanchez Prefractionation of Complex Protein Mixture for 2-D PAGE Using Reversed-Phase Liquid Chromatography Volker Badock and Albrecht Otto Fractionation of Complex Proteomes by Microscale Solution Isoelectrofocusing Using ZOOM(TM) IEF Fractionators to Improve Protein Profiling Xun Zuo, Ki-Boom Lee, and David W. Speicher Large-Format 2-D Polyacrylamide Gel Electrophoresis Henry Brzeski, Stephen Russell, Anthony G. Sullivan, Richard I. Somiari, and Craig D. Shiver Analysis of Membrane Proteins by Two-Dimensional Gels MichaelFountoulakis 2-D PAGE of High-Molecular-Mass Proteins Masamichi Oh-Ishi and Tadakazu Maeda Using Ultra-Zoom Gels for High-Resolution Two-Dimensional Polyacrylamide Gel Electrophoresis Sjouke Hoving, Hans Voshol, and Jan van Oostrum NEpHGE and pI Strip Proteomic 2-D Gel Electrophoretic Mapping of Lipid-Rich Membranes Steven E. Pfeiffer, Yoshihide Yamaguchi, Cecilia B. Marta, Rashmi Bansal, and Christopher M. Taylor Silver Staining of 2-D Gels Julia Poland, Thierry Rabilloud, and Pranav Sinha Zn2+ Reverse Staining Technique Carlos Fernandez-Patron Multiplexed Proteomics Technology for the Fluorescence Detection of Glycoprotein Levels and Protein Expression Levels Using Pro-Q(R) Emerald and SYPRO(R) Ruby Dyes Birte Schulenberg and Wayne F. Patton Multiplexed Proteomics Technology for the Fluorescence Detection of Phosphorylation and Protein Expression Levels Using Pro-Q(R) Diamond and SYPRO(R) Ruby Dyes Birte Schulenberg, Terrie Goodman, Thomas H. Steinberg, and Wayne F. Patton Sensitive Quantitative Fluorescence Detection of Proteins in Gels Using SYPRO(R) Ruby Protein Gel Stain Birte Schulenberg, Nancy Ahnert, and Wayne F. Patton Rapid, Sensitive Detection of Proteins in Minigels With Fluorescent Dyes: Coomassie Fluor Orange, SYPRO(R) Orange, SYPRO Red, and SYPRO Tangerine Protein Gel Stains Thomas H. Steinberg, Courtenay R. Hart, and Wayne F. Patton Differential In-Gel Electrophoresis in a High-Throughput Environment Richard I. Somiari, Stephen Russell, Stella B. Somiari, Anthony G. Sullivan, Darrell L. Ellsworth, Henry Brzeski, and Craig D. Shriver Statistical Analysis of 2-D Gel Patterns Francoise Seillier-Moiseiwitsch 2-DE Databases on the World Wide Web Christine Hoogland, Khaled Mostaguir, and Ron D. Appel Computer Analysis of 2-D Images Patricia M. Palagi,

The proteomics protocols handbook

The sequence of a genome contains the plans of the possible life of an organism, but implementation of genetic information depends on the functions of the proteins and nucleic acids that it encodes. Many individual proteins of known sequence and structure present challenges to the understanding of their function. In particular, a number of genes responsible for diseases have been identified but their specific functions are unknown. Whole-genome sequencing projects are a major source of proteins of unknown function. Annotation of a genome involves assignment of functions to gene products, in most cases on the basis of amino-acid sequence alone. 3D structure can aid the assignment of function, motivating the challenge of structural genomics projects to make structural information available for novel uncharacterized proteins. Structure-based identification of homologues often succeeds where sequence-alone-based methods fail, because in many cases evolution retains the folding pattern long after sequence similarity becomes undetectable. Nevertheless, prediction of protein function from sequence and structure is a difficult problem, because homologous proteins often have different functions. Many methods of function prediction rely on identifying similarity in sequence and/or structure between a protein of unknown function and one or more well-understood proteins. Alternative methods include inferring conservation patterns in members of a functionally uncharacterized family for which many sequences and structures are known. However, these inferences are tenuous. Such methods provide reasonable guesses at function, but are far from foolproof. It is therefore fortunate that the development of whole-organism approaches and comparative genomics permits other approaches to function prediction when the data are available. These include the use of protein-protein interaction patterns, and correlations between occurrences of related proteins in different organisms, as indicators of functional properties. Even if it is possible to ascribe a particular function to a gene product, the protein may have multiple functions. A fundamental problem is that function is in many cases an ill-defined concept. In this article we review the state of the art in function prediction and describe some of the underlying difficulties and successes.

/pdf/prediction-of-protein-function-from-protein-sequence-and-1x3ddufwc1.pdf

Prediction of protein function from protein sequence and structure.

Heterotrimeric G proteins have a crucial role as molecular switches in signal transduction pathways mediated by G-protein-coupled receptors. Extracellular stimuli activate these receptors, which then catalyse GTP-GDP exchange on the G protein alpha-subunit. The complex series of interactions and conformational changes that connect agonist binding to G protein activation raise various interesting questions about the structure, biomechanics, kinetics and specificity of signal transduction across the plasma membrane.

Heterotrimeric G protein activation by G-protein-coupled receptors

Regulators of G protein signaling (RGS) and RGS-like proteins are a family (>30 members) of highly diverse, multifunctional signaling proteins that bind directly to activated G alpha subunits. Family members are defined by a shared RGS domain, which is responsible for G alpha binding and markedly stimulates the GTPase activity of G alpha subunits leading to their deactivation and termination of downstream signals. Although much has been learned in recent years about the biochemistry of RGS/G alpha interactions, considerably less is known about the broader cellular roles and regulation of RGS proteins. Recent findings indicate that cellular mechanisms such as covalent modification, alternative gene splicing, and protein processing can dictate the activity and subcellular localization of RGS proteins. Many family members also directly link G proteins to a growing list of signaling proteins with diverse cellular roles. New findings indicate that RGS proteins act not as dedicated inhibitors but, rather, as tightly regulated modulators and integrators of G protein signaling. In some cases, RGS proteins modulate the lifetime and kinetics of both slow-acting (e.g., Ca(2+) oscillations) and fast-acting (e.g., ion conductances, phototransduction) signaling responses. In other cases, RGS proteins integrate G proteins with signaling pathways linked to such diverse cellular responses as cell growth and differentiation, cell motility, and intracellular trafficking. These and other recent studies with animal model systems indicate that RGS proteins play important roles in both physiology and disease. Recognition of the central functions these proteins play in vital cellular processes has focused our attention on RGS proteins as exciting new candidates for therapeutic intervention and drug development.

Cellular Regulation of RGS Proteins: Modulators and Integrators of G Protein Signaling

A multitude of heptahelical receptors use heterotrimeric G proteins to transduce signals to specific effector target molecules. The G protein transducin, Gt, couples photon-activated rhodopsin with the effector cyclic GMP phosophodiesterase (PDE) in the vertebrate phototransduction cascade. The interactions of the Gt alpha-subunit (alpha(t)) with the inhibitory PDE gamma-subunit (PDEgamma) are central to effector activation, and also enhance visual recovery in cooperation with the GTPase-activating protein regulator of G-protein signalling (RGS)-9 (refs 1-3). Here we describe the crystal structure at 2.0 A of rod transducin alpha x GDP x AlF4- in complex with the effector molecule PDEgamma and the GTPase-activating protein RGS9. In addition, we present the independently solved crystal structures of the RGS9 RGS domain both alone and in complex with alpha(t/i1) x GDP x AlF4-. These structures reveal insights into effector activation, synergistic GTPase acceleration, RGS9 specificity and RGS activity. Effector binding to a nucleotide-dependent site on alpha(t) sequesters PDEgamma residues implicated in PDE inhibition, and potentiates recruitment of RGS9 for hydrolytic transition state stabilization and concomitant signal termination.

Structural determinants for regulation of phosphodiesterase by a G protein at 2.0 Å

A critical challenge of structural genomics is to extract functional information from protein structures. We present an example of how this may be accomplished using the Evolutionary Trace (ET) method in the context of the regulators of G protein signaling (RGS) family. We have previously applied ET to the RGS family and identified a novel, evolutionarily privileged site on the RGS domain as important for regulating RGS activity. Here we confirm through targeted mutagenesis of RGS7 that these ET-identified residues are critical for RGS domain regulation and are likely to function as global determinants of RGS function. We also discuss how the recent structure of the complex of RGS9, Gt/i1α–GDP–AlF4− and the effector subunit PDEγ confirms their contact with the effector–G protein interface, forming a structural pathway that communicates from the effector-contacting surface of the G protein and RGS catalytic core domain to the catalytic interface between Gα and RGS. These results demonstrate the effectiveness of ET for identifying binding sites and efficiently focusing mutational studies on their key residues, thereby linking raw sequence and structure data to functional information.

Prediction and confirmation of a site critical for effector regulation of RGS domain activity.

Heterotrimeric G proteins relay extracellular cues from heptahelical transmembrane receptors to downstream effector molecules. Composed of an α subunit with intrinsic GTPase activity and a βγ heterodimer, the trimeric complex dissociates upon receptor-mediated nucleotide exchange on the α subunit, enabling each component to engage downstream effector targets for either activation or inhibition as dictated in a particular pathway. To mitigate excessive effector engagement and concomitant signal transmission, the Gα subunit's intrinsic activation timer (the rate of GTP hydrolysis) is regulated spatially and temporally by a class of GTPase accelerating proteins (GAPs) known as the regulator of G protein signaling (RGS) family. The array of G protein-coupled receptors, Gα subunits, RGS proteins and downstream effectors in mammalian systems is vast. Understanding the molecular determinants of specificity is critical for a comprehensive mapping of the G protein system. Here, we present the 2.9 A crystal structure of the enigmatic, neuronal G protein Gαo in the GTP hydrolytic transition state, complexed with RGS16. Comparison with the 1.89 A structure of apo-RGS16, also presented here, reveals plasticity upon Gαo binding, the determinants for GAP activity, and the structurally unique features of Gαo that likely distinguish it physiologically from other members of the larger Gαi family, affording insight to receptor, GAP and effector specificity.

https://www.pnas.org/content/pnas/105/17/6243.full.pdf

Michele A. Kercher

Papers

Structural determinants for regulation of phosphodiesterase by a G protein at 2.0 Å

Prediction and confirmation of a site critical for effector regulation of RGS domain activity.

Molecular architecture of Gαo and the structural basis for RGS16-mediated deactivation