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Figure 1.
Fig. 1. The structure of G[s][  ]·GTP
 S. (A) A
dimer of G[s][ ]·GTP
S was
observed in the asymmetric unit of the crystals and^ is depicted
here as a ribbon and coil diagram looking down the^
noncrystallographic twofold axis. The 16 phosphate anions are^
drawn as red tetrahedrons. Most of the anions bind within a
groove^ at the dimer interface between the  5 helices.
The two phosphate^ anions that bind near the NH[2]-termini of
each molecule of G[s][ ]form
crystal contacts. GTP S (yellow)
and Mg2+ (black) are represented by ball-and-stick models and
are located^ in the nucleotide binding pocket. Helices are
green,  strands
are purple, and coils are gray. This and the other ribbon
diagrams were generated with MOLSCRIPT (40) and rendered with
RASTER3D^ (41). (B) Superposition of G[i][ ](transparent
rose) on the structure of G[s][ ]·GTP
S (solid
gray). Only the nucleotide^ bound to G[s][ ]is
shown. The approximate locations of two of^ the three major
insertions in the G[s][ ]sequence
relative to G[i][ ](i2
and i3) are indicated in white (see text). The two proteins
superimpose with a rmsd of 1.0 Å for 260 C atom
pairs. Their structures are essentially identical at the GTP
binding site and are most divergent in various loops at the
periphery of the molecule, most notably at the 3- 5 and 4- 6 loops.
(C) Sequence alignment of representative proteins from three G[
]subfamilies:
bovine G[s][ ](Protein
Information Resource accession number A23813), murine G[q][ ](A38414),
and bovine G[i][ ][1]^
(A23631) (42). Secondary structure has been assigned on the^
basis of the structures of G[s][ ]and
G[i][ ][1]·GTP
S (7). The
three conformationally flexible switch elements are indicated^
by red blocks. The arrow marks the site in G[s][ ]at
which the^ long and short splice variants differ in length by 14
amino acids. Green amino acid letters indicate residues in G[s][
]that
contact adenylyl cyclase, whereas red amino acid letters
indicate potential adenylyl cyclase binding residues in G[i][
]identified
by alanine-scanning mutagenesis (21). The general locations of
the i1, i2, and i3^ insertions are also indicated.
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