G protein

Examples of G protein in the following topics:

• Types of Receptors

  • G-protein-linked receptors bind a ligand and activate a membrane protein called a G-protein.
  • All G-protein-linked receptors have seven transmembrane domains, but each receptor has its own specific extracellular domain and G-protein-binding site.
  • Once the G-protein binds to the receptor, the resultant shape change activates the G-protein, which releases GDP and picks up GTP.
  • One or both of these G-protein fragments may be able to activate other proteins as a result.
  • Heterotrimeric G proteins have three subunits: α, β, and γ.

• Plasma Membrane Hormone Receptors

  • When a hormone binds to its membrane receptor, a G protein that is associated with the receptor is activated.
  • G proteins are proteins separate from receptors that are found in the cell membrane.
  • When a hormone is not bound to the receptor, the G protein is inactive and is bound to guanosine diphosphate, or GDP.
  • After binding, GTP is hydrolyzed by the G protein into GDP and becomes inactive .
  • The activated G protein in turn activates a membrane-bound enzyme called adenylyl cyclase.

• Ionotropic and Metabotropic Receptors

  • Although both ionotropic and metabotropic receptors are activated by neurotransmitters, ionotropic receptors are channel-linked while metabotropic receptors initiate a cascade of molecules via G-proteins.
  • Two types of membrane-bound receptors are activated with the binding of neurotransmitters: ligand-gated ion channels (LGICs) inotropic receptors and metabotropic G- protein coupled receptors.
  • Metabotropic receptors are a subtype of membrane receptors that do not form an ion channel pore but use signal transduction mechanisms, often G proteins, to activate a series of intracellular events using second messenger chemicals.
  • The metabotropic G protein-coupled receptors have seven hydrophobic transmembrane domains.
  • When the neurotransmitter binds to the receptor, there is an activation via the G-protein that later activates the  secondary messengers.

• Respiratory Syncytial Virus Infection

  • RSV has ten genes encoding 11 proteins.
  • SH, G and F form the viral coat.
  • The G protein is a surface protein; it functions as the attachment protein, the protein which attaches the virus to target cells.
  • The F protein is another important surface protein.
  • Antibodies directed at the F protein are neutralizing.

• Phage Display

  • The bacteria process the new gene so that a new protein or peptide is made.
  • This protein or peptide is exposed on the phage surface.
  • The related proteins keep most of the physical and chemical properties of their parent protein.
  • Phage display technology is advantageous in many applications including selection of inhibitors for the active and allosteric sites of enzymes, receptor agonists and antagonists, and G-protein binding modulatory peptides.
  • Phage display is also used in epitope mapping and analysis of protein-protein interactions.

• The Incorporation of Nonstandard Amino Acids

  • It is one of the later steps in protein biosynthesis, and thus gene expression, for many proteins.
  • During protein synthesis, 20 different amino acids can be incorporated to become a protein.
  • After translation, the posttranslational modification of amino acids extends the range of functions of the protein by attaching it to other biochemical functional groups (such as acetate, phosphate, various lipids, and carbohydrates), changing the chemical nature of an amino acid (e.g., citrullination), or making structural changes (e.g., formation of disulfide bridges).
  • Those either are not found in proteins (e.g., carnitine, GABA), or are not produced directly and in isolation by standard cellular machinery (e.g., hydroxyproline and selenomethionine).
  • Some nonstandard amino acids are not found in proteins.

• Whole-Genome DNA-Binding Analysis

  • It has also become a widely-used method for genome-wide localization of protein-DNA interactions.
  • The first step in the ChIP-Chip procedure is to fix protein-DNA interactions in living cells by chemical crosslinking.
  • This extract is then subjected to immunoprecipitation (IP) with a specific antibody against the protein of interest.
  • DNA bound by the protein will be coprecipitated and enriched, compared to DNA not bound by the respective protein.
  • To facilitate immunoprecipitation and subsequent washing, antibodies are usually coupled to either agarose- or magnetic beads via protein A or G.

• Epigenetic Alterations in Cancer

  • Common in cancer cells, silencing genes, which occur through epigenetic mechanisms, include modifications to histone proteins and DNA.
  • Mechanisms of epigenetic silencing of tumor suppressor genes and activation of oncogenes include: alteration in CpG island methylation patterns, histone modifications, and dysregulation of DNA binding proteins.
  • Silencing genes through epigenetic mechanisms is very common in cancer cells and include modifications to histone proteins and DNA that are associated with silenced genes.
  • In cancer cells, the DNA in the promoter region of silenced genes is methylated on cytosine DNA residues in CpG islands, genomic regions that contain a high frequency of CpG sites, where a cytosine nucleotide occurs next to a guanine nucleotide .
  • Mechanisms can include modifications to histone proteins and DNA associated with these silencing genes.

• Mapping Protein-Protein Interactions

  • Mapping protein-protein interactions gives us a better understanding of molecular mechanisms inside the cell.
  • The protein complexes formed could be stable (proteins interact for a prolonged period of time) or transient (proteins interact for a brief period of time).
  • If the two proteins do interact the bait recruits the prey to a specific cellular location where it can stimulate a detectable output (e.g., gene activation).
  • The tag serves as a tool to purify the bait protein and associated proteins by affinity chromatography.
  • Principle of the bait and prey method for the study of protein-protein interaction.

• Two-Hybrid Analysis

  • Several methodologies exist to study the interaction of proteins in vivo.
  • The yeast two-hybrid screening system is an effective and quick tool for the in vivo study of protein–protein interaction both in prokaryotes and eukaryotes.
  • If the two proteins do interact the bait recruits the prey to a specific cellular location where it can stimulate a detectable output (e.g., gene activation).
  • It is therefore impossible to use them to study the protein–protein interactions between insoluble integral membrane proteins.
  • These fused proteins are called the bait and prey, respectively.