PDBsum entry 1azt

Hydrolase

PDB id: 1azt

Contents

Protein chains

335 a.a. *

Ligands

PO4 ×16

GSP ×2

Metals

_MG ×2

Waters ×57

* Residue conservation analysis

References listed in PDB file

Key reference

Title

Crystal structure of the adenylyl cyclase activator gsalpha.

Authors

R.K.Sunahara, J.J.Tesmer, A.G.Gilman, S.R.Sprang.

Ref.

Science, 1997, 278, 1943-1947. [DOI no: 10.1126/science.278.5345.1943]

PubMed id

9395396

Abstract

The crystal structure of Gsalpha, the heterotrimeric G protein alpha subunit that stimulates adenylyl cyclase, was determined at 2.5 A in a complex with guanosine 5'-O-(3-thiotriphosphate) (GTPgammaS). Gsalpha is the prototypic member of a family of GTP-binding proteins that regulate the activities of effectors in a hormone-dependent manner. Comparison of the structure of Gsalpha.GTPgammaS with that of Gialpha.GTPgammaS suggests that their effector specificity is primarily dictated by the shape of the binding surface formed by the switch II helix and the alpha3-beta5 loop, despite the high sequence homology of these elements. In contrast, sequence divergence explains the inability of regulators of G protein signaling to stimulate the GTPase activity of Gsalpha. The betagamma binding surface of Gsalpha is largely conserved in sequence and structure to that of Gialpha, whereas differences in the surface formed by the carboxyl-terminal helix and the alpha4-beta6 loop may mediate receptor specificity.

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.

Figure 2.
Fig. 2. Superposition of the putative effector binding loops ( 2- 4, 3- 5, and 4- 6) and the 5 helix from G[s][ ]onto G[i][ ]^(42). The side chains from residues of G[s][ ]are drawn as^ stick models with the use of conventional coloring. The backbone^ and side chains of G[i][ ]are illustrated in transparent rose.^ The model of G[i][ ]is derived from the structure of the G[i][ ][1]·RGS4^ complex (17), which has a completely ordered 5 helix. The superposition^ is essentially the same as that shown in Fig. 1B. The 2- 4 loops^ of each subunit are essentially identical. The 3- 5 loop of^ G[s][ ], although structurally similar to that of G[i][ ], is^ rotated downward in the figure. This rotation creates a hydrophobic^ pocket on the back side of the sheet, which is filled by the^ side chain of Met^386 from the 5 helix, and moves the residue at position 282 in G[s][ ]^toward the conserved Phe^238. In the G[s] subfamily, residue 282 is a leucine, which helps to^ accommodate the shift of the 3- 5 loop. The 4- 6 loop of G[s][ ]^is longer than and shares no sequence identity with its counterpart^ in G[i][ ]. The 3- 5 and 4- 6 loops are supported by a stacking^ interaction between Trp^277 and His^357, both of which are invariant in the G[s] subfamily. The 5 helix^ of G[s][ ]is bent, whereas that of G[i][ ]extends straight into^ solvent. The large differences observed in the 4- 6 and 5 structures^ may help account for receptor specificity among closely related^ subunits.

The above figures are reprinted by permission from the AAAs: Science (1997, 278, 1943-1947) copyright 1997.