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PDBsum entry 2z73
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Membrane protein
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PDB id
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2z73
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References listed in PDB file
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Key reference
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Title
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Crystal structure of squid rhodopsin.
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Authors
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M.Murakami,
T.Kouyama.
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Ref.
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Nature, 2008,
453,
363-367.
[DOI no: ]
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PubMed id
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Abstract
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Invertebrate phototransduction uses an inositol-1,4,5-trisphosphate signalling
cascade in which photoactivated rhodopsin stimulates a G(q)-type G protein, that
is, a class of G protein that stimulates membrane-bound phospholipase Cbeta. The
same cascade is used by many G-protein-coupled receptors, indicating that
invertebrate rhodopsin is a prototypical member. Here we report the crystal
structure of squid (Todarodes pacificus) rhodopsin at 2.5 A resolution. Among
seven transmembrane alpha-helices, helices V and VI extend into the cytoplasmic
medium and, together with two cytoplasmic helices, they form a rigid protrusion
from the membrane surface. This peculiar structure, which is not seen in bovine
rhodopsin, seems to be crucial for the recognition of G(q)-type G proteins. The
retinal Schiff base forms a hydrogen bond to Asn 87 or Tyr 111; it is far from
the putative counterion Glu 180. In the crystal, a tight association is formed
between the amino-terminal polypeptides of neighbouring monomers; this
intermembrane dimerization may be responsible for the organization of
hexagonally packed microvillar membranes in the photoreceptor rhabdom.
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Figure 1.
Figure 1: Schematic diagram of the topology of squid rhodopsin.
 -Helices
are denoted by black rectangles, 3[10] helices as grey
rectangles and  -strands
as open arrows. In the current structural model, the polypeptide
chain is traced from Glu 9 to Glu 358 (shown by black letters).
Grey letters indicate residues that are truncated or disordered.
Retinal bound to Lys 305 and palmitoyl bound to Cys 337 are
included in the structural model. A disulphide bond is formed
between Cys 108 and Cys 186. The counterion Glu 180, residues
within 3.5 Å of the Schiff base and within 4 Å of
the retinal polyene chain are circled by red, purple and green
lines, respectively. Positively and negatively ionizable
residues are marked with blue and pink circles, respectively.
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Figure 3.
Figure 3: Structural comparisons between squid rhodopsin and
bovine rhodopsin. Squid rhodopsin is denoted by brown and
magenta; bovine rhodopsin by cyan and blue.
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The above figures are
reprinted
by permission from Macmillan Publishers Ltd:
Nature
(2008,
453,
363-367)
copyright 2008.
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