Monomeric GPCR as a Functional Unit Marc Chabre and Marc le Maire

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Monomeric GPCR as a Functional UnitMarc Chabre
and Marc le Maire

• Presented by Naveena Sivaram

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Overview

• Challenges monomer vs dimer concept
• Study found to strengthen support - monomeric
  model of rhodopsin and other class 1a GPCRs
• Proposed a molecular model for functional
  coupling of Rhodopsin monomeric model with
  G-Protein Heterotrimer

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GPCR encoded for gt1000 genes, represent app.1 of
human gemone
(A) GPCRs have a central common core made of
seven transmembrane helices (TM-I to -VII)
connected by three intracellular (i1, i2, i3) and
three extracellular (e1, e2, e3) loops. The
diversity of messages which activate those
receptors is an illustration of their
evolutionary success. (B) Illustration of the
central core of rhodopsin. The core is viewed
from the cytoplasm. The length and orientation of
the TMs are deduced from the two-dimensional
crystal of bovine and frog rhodopsin (Unger et
al., 1997). The N- and C-terminal of i2
(including the DRY sequence) and i3 are included
in TM-III and -VI. The core is represented under
its 'active conformation'. The TM-VI and -VII
lean out of the structure, the TM-VI turn by 30
on its axis (clockwise as viewed from the
cytoplasm) (Bourne, 1997). This opens a cleft in
the central core in which G proteins can find
their way. i2 and i3 loops are the two main loops
engaged in G protein recognition and activation.
EMBO J. 18 1723-1729 (1999)
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Family 1 contains most GPCRs including receptors
for odorants, small ligands, peptide hormone and
glycoprotein. Disulphide bridge connects e1 and
e2 and palmitoylated cysteine in C-terminal.
Family 2 GPCRs have relative long N-terminal that
contains several cysteines (network of disulphide
bridge). Examples include glucagon, GnRH and PTH
receptors.
Family 3 GPCRs have very large span N-terminal
sequences and C-terminal tail. The ligand binding
domain is located in the N-treminus. The i3 loop
is short and highly conserved. Representative
samples are mGluR, Ca2-sensing and GABA-B
receptor.
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GTP binding protein (G-protein)

• Definition binding the guanine nucleotides (GTP
  and GDP) and process intrinsic GTPase activity
• physiological meaning essential role in signal
  transduction, vesicle transport, cytoskeletal
  assembly and cell growth
• classification heterotrimer G protein (abg) and
  small G proteins (ras, raf, rho or rab protein)

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Consequence of receptor activation

• a. Alteration in GPCR TM domain TM3 and TM6 spin
  and conformation change
• b. G-protein dissociation (? separate from ??)
• c. signal transfer to cytoplasmic effectors (
  adenylyl cyclase, PLC etc. )
• d. Activation of effectors
• e. signal generation downwards

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Early Studies

• Hydrodynamic studies with Triton X100 by Osborne
  etal
• Small angle neutron expt.. in polyoxyethylene
  glycol
• Late biochemist Hermann Kuhn functional
  interaction between R Gt???
• Though one exception for monomeric model was
  stated in Nature review article in 1981.

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Recent Studies

• Medina etal..
• oligomeric state using cross-linking expts
• Dm solubilized rhodopsin was dimeric
• Jastrzebska etal cross linking expts
• Park Wells M2 muscranic cholinergic receptor
• Mesnier and Baneres cross linking expts

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Evidences

• None of crystallographic studies suggest
  existence of a dimer
• Structure is of an inactive rhodopsin
• Inconsistencies in the structure of Gt???

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Molecular Model of Interaction RhodopsinMonomer
and a Gt???

• Park etal argues that Gt is larger than rhodopsin
  monomer
• Gt? and Gt? C-terminal peptides doesnt interact
  with inactive rhodopsin interacts with photo
  activated rhodopsin at two different sites.
• Proposed Model by Marc etal.

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Figure Models for a rhodopsin monomer in the
lipid membrane, a transducin heterotrimer
attached to the lipid membrane, and a complex
between a photoactivated rhodopsin and a
transducin.
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Future Studies

• Crystallization of a complex with R and
  nucleotide free Gt???
• Obtaining a high resolution structure

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Thank You