Transmembrane and GPCR Mohammed Mohammed Khan PhD Scholar- Department of Biochemistry King Abdul-Aziz University

Receptors are the gates for incoming signals in that they specifically receive the signal and are thereby activated for further signal transduction Receptors generally trigger very rapid physiological or biochemical responses.

Peripheral membrane proteins are membrane proteins that adhere only temporarily to the biological membrane with which they are associated. These proteins attach to integral membrane proteins, or penetrate the peripheral regions of the lipid bilayer. Peripheral membrane proteins tend to collect in the water-soluble component, or fraction, of all the proteins extracted during a protein purification procedure 1.Interaction by an amphipathic α-helix parallel to the membrane plane (in-plane membrane helix) 2.Interaction by a hydrophobic loop 3.Interaction by a covalently bound membrane lipid 4.Electrostatic or ionic interactions with membrane lipids

Transmembrane receptors are proteins that span the phospholipid bilayer of the cell membrane. It contains, an extracellular domain, a transmembrane domain and an intracellular or cytosolic domain can be differentiated within the structure The signaling molecule binds on the extracellular side to the receptor and get activated. Ligand controlled transmembrane receptor Ligand – gated ion channels Voltage- gated ion channels

The Extracellular Domain of Transmembrane Receptors:In many receptors, the extracellular domain contains the ligand-binding site. Glycosylation sites, i.e. attachment sites for carbohydrate residues, are also located nearby in the extracellular domain a) Representation of the most important functional domains of transmembrane receptors Examples of subunit structures. Examples of structures of the transmembrane domains of receptors

G protein coupled receptor Gpcr transmembrane receptors that receive signals and conduct them into the cell interior, the G protein-coupled receptors form the largest single family. In vertebrates, more than 1000 different G protein-coupled receptors are found that may be activated by extracellular ligands or sensory signals.( i.e adrenaline and noradrenaline, histamine, serotonin, lipid derivatives, nucleotides, retinal derivatives., etc) A characteristic structural feature of the G protein-coupled receptors is the presence of 7 transmembrane helices. For the vast majority of 7-helix transmembrane receptors the next downstream located signaling protein is a heterotrimeric G protein

Dimerization of GPCRs Although the GPCRs were generally believed to function as monomeric entities, there is now increasing evidence that G protein- coupled receptors may form functional dimers in vivo. Biochemical and biophysical studies suggest that, e.g., the b2 adrenergic receptor exists as a constitutive dimer in the cell (Angers et al., 2002).

The GTPase Superfamily: General Functions and Mechanism proteins of the GTPase superfamily are found in all plant, bacterial and animal systems. The following examples illustrate the central functions of the regulatory GTPases in the cell. Regulatory GTPases are involved in – protein biosynthesis on ribosomes – signal transduction at membranes – visual perception – sense of smell and taste – control of differentiation and cell division – translocation of proteins through membranes – transport of vesicles in the cell.

Fig: Activation of the G alpha subunit of a G-protein-coupled receptor In unstimulated cells, the state of G alpha (orange circles) is defined by its interaction with GDP, G beta-gamma (purple circles), and a G-protein-coupled receptor (GPCR; light green loops). Upon receptor stimulation by a ligand called an agonist, the state of the receptor changes. G alpha dissociates from the receptor and G beta-gamma, and GTP is exchanged for the bound GDP, which leads to G alpha activation. G alpha then goes on to activate other molecules in the cell. © 2002 Nature Publishing Group Li, J. et al. The Molecule Pages database. Nature 420,

2005 Nature Publishing Group Leurs, R. et al. The histamine H3 receptor: from gene cloning to H3 receptor drugs. Nature Reviews Drug Discovery 4, (2005). All rights reserved

Receptor desensitization: phosphorylation, arrestin binding and internalization. The activated, agonist-bound receptor is phosphorylated on the cytoplasmic region by a G protein-coupled receptor protein kinase (GRK). The phosphate residues serve as attachment sites for b-arrestin which has protein kinases of the MAPK cascade associated. This serves as a trigger for internalization of the receptor to endosomes. The receptor may now be dephosphorylated and transported back to the cell membrane Pitcher et al., 1998

Regulators of G Proteins: Phosducin and RGS Proteins Regulation of G protein-coupled receptors and of G proteins. Signal transduction by activated G protein-coupled receptors (GPCRs) is mainly regulated by phosphorylation via GPCR kinases (GRKs) leading to downregulation and desensitization. Signaling by GaGTP can be negatively controlled by regulators of G protein signaling (RGS) and by the effector proteins themselves. Anegative control of bc-complex signaling is mediated by phosducin.