Paul B. Sigler

The crystal structure of the asymmetric GroEL-GroES-(ADP)7 chaperonin complex.

Chaperonins assist protein folding with the consumption of ATP. They exist as multi-subunit protein assemblies comprising rings of subunits stacked back to back. In Escherichia coli, asymmetric intermediates of GroEL are formed with the co-chaperonin GroES and nucleotides bound only to one of the seven-subunit rings (the cis ring) and not to the opposing ring (the trans ring). The structure of the GroEL–GroES–(ADP)7 complex reveals how large en bloc movements of the cis rings intermediate and apical domains enable bound GroES to stabilize a folding chamber with ADP confined to the cis ring. Elevation and twist of the apical domains double the volume of the central cavity and bury hydrophobic peptide-binding residues in the interface with GroES, as well as between GroEL subunits, leaving a hydrophilic cavity lining that is conducive to protein folding. An inward tilt of the cis equatorial domain causes an outward tilt in the trans ring that opposes the binding of a second GroES. When combined with new functional results, this negative allosteric mechanism suggests a model for an ATP-driven folding cycle that requires a double toroid.

Interfacial catalysis: the mechanism of phospholipase A2

A chemical description of the action of phospholipase A2 (PLA2) can now be inferred with confidence from three high-resolution x-ray crystal structures. The first is the structure of the PLA2 from the venom of the Chinese cobra (Naja naja atra) in a complex with a phosphonate transition-state analogue. This enzyme is typical of a large, well-studied homologous family of PLA2S. The second is a similar complex with the evolutionarily distant bee-venom PLA2. The third structure is the uninhibited PLA2 from Chinese cobra venom. Despite the different molecular architectures of the cobra and bee-venom PLA2s, the transition-state analogue interacts in a nearly identical way with the catalytic machinery of both enzymes. The disposition of the fatty-acid side chains suggests a common access route of the substrate from its position in the lipid aggregate to its productive interaction with the active site. Comparison of the cobra-venom complex with the uninhibited enzyme indicates that optimal binding and catalysis at the lipid-water interface is due to facilitated substrate diffusion from the interfacial binding surface to the catalytic site rather than an allosteric change in the enzymes structure. However, a second bound calcium ion changes its position upon the binding of the transition-state analogue, suggesting a mechanism for augmenting the critical electrophile.

Crystal structure of bee-venom phospholipase A2 in a complex with a transition-state analogue

The 2.0 angstroms crystal structure of a complex containing bee-venom phospholipase A2 (PLA2) and a phosphonate transition-state analogue was solved by multiple isomorphous replacement. The electron-density map is sufficiently detailed to visualize the proximal sugars of the enzymes N-linked carbohydrate and a single molecule of the transition-state analogue bound ot its active center. Although bee-venom PLA2 does not belong to the large homologous Class I/II family that encompasses most other well-studied PLA2s, there is segmental sequence similarity and conservation of many functional substructures. Comparison of the bee-venom enzyme with other phospholipase structures provides compelling evidence for a common catalytic mechanism.

Distinct actions of cis and trans ATP within the double ring of the chaperonin GroEL

The chaperonin GroEL is a double-ring structure with a central cavity in each ring that provides an environment for the efficient folding of proteins when capped by the co-chaperone GroES in the presence of adenine nucleotides. Productive folding of the substrate rhodanese has been observed in cis ternary complexes, where GroES and polypeptide are bound to the same ring, formed with either ATP, ADP or non-hydrolysable ATP analogues,, suggesting that the specific requirement for ATP is confined to an action in the trans ring that evicts GroES and polypeptide from the cis side. We show here, however, that for the folding of malate dehydrogenase and Rubisco there is also an absolute requirement for ATP in the cis ring, as ADP and AMP-PNP are unable to promote folding. We investigated the specific roles of binding and hydrolysis of ATP in the cis and trans rings using mutant forms of GroEL that bind ATP but are defective in its hydrolysis. Binding of ATP and GroES in cis initiated productive folding inside a highly stable GroEL–ATP–GroES complex. To discharge GroES and polypeptide, ATP hydrolysis in the cis ring was required to form a GroEL–ADP–GroES complex with decreased stability, priming the cis complex for release by ATP binding (without hydrolysis) in the trans ring. These observations offer an explanation of why GroEL functions as a double-ring complex.

A Model for Arrestin’s Regulation: The 2.8 Å Crystal Structure of Visual Arrestin

G protein-coupled signaling is utilized by a wide variety of eukaryotes for communicating information from the extracellular environment. Signal termination is achieved by the action of the arrestins, which bind to activated, phosphorylated G protein-coupled receptors. We describe here crystallographic studies of visual arrestin in its basal conformation. The salient features of the structure are a bipartite molecule with an unusual polar core. This core is stabilized in part by an extended carboxy-terminal tail that locks the molecule into an inactive state. In addition, arrestin is found to be a dimer of two asymmetric molecules, suggesting an intrinsic conformational plasticity. In conjunction with biochemical and mutagenesis data, we propose a molecular mechanism by which arrestin is activated for receptor binding.

Crystal structure of beta-arrestin at 1.9 A: possible mechanism of receptor binding and membrane Translocation.

BACKGROUND Arrestins are responsible for the desensitization of many sequence-divergent G protein-coupled receptors. They compete with G proteins for binding to activated phosphorylated receptors, initiate receptor internalization, and activate additional signaling pathways. RESULTS In order to understand the structural basis for receptor binding and arrestins function as an adaptor molecule, we determined the X-ray crystal structure of two truncated forms of bovine beta-arrestin in its cytosolic inactive state to 1.9 A. Mutational analysis and chimera studies identify the regions in beta-arrestin responsible for receptor binding specificity. beta-arrestin demonstrates high structural homology with the previously solved visual arrestin. All key structural elements responsible for arrestins mechanism of activation are conserved. CONCLUSIONS Based on structural analysis and mutagenesis data, we propose a previously unappreciated part in beta-arrestins mode of action by which a cationic amphipathic helix may function as a reversible membrane anchor. This novel activation mechanism would facilitate the formation of a high-affinity complex between beta-arrestin and an activated receptor regardless of its specific subtype. Like the interaction between beta-arrestins polar core and the phosphorylated receptor, such a general activation mechanism would contribute to beta-arrestins versatility as a regulator of many receptors.

The crystal structure of a GroEL/peptide complex: plasticity as a basis for substrate diversity.

The chaperonin GroEL is a double toriodal assembly that with its cochaperonin GroES facilitates protein folding with an ATP-dependent mechanism. Nonnative conformations of diverse protein substrates bind to the apical domains surrounding the opening of the double toroids central cavity. Using phage display, we have selected peptides with high affinity for the isolated apical domain. We have determined the crystal structures of the complexes formed by the most strongly bound peptide with the isolated apical domain, and with GroEL. The peptide interacts with the groove between paired alpha helices in a manner similar to that of the GroES mobile loop. Our structural analysis, combined with other results, suggests that various modes of molecular plasticity are responsible for tight promiscuous binding of nonnative substrates and their release into the shielded cis assembly.

The 2.4 Å crystal structure of the bacterial chaperonin GroEL complexed with ATPγS

GroEL is a bacterial chaperonin of 14 identical subunits required to help fold newly synthesized proteins. The crystal structure of GroEL with ATPγS bound to each subunit shows that ATP binds to a novel pocket, whose primary sequence is highly conserved among chaperonins. Interaction of Mg2+ and ATP involves phosphate oxygens of the α-, β- and γ-phosphates, which is unique for known structures of nucleotide-binding proteins. Although bound ATP induces modest conformational shifts in the equatorial domain, the stereochemistry that functionally coordinates GroELs affinity for nucleotides, polypeptide, and GroES remains uncertain.

The 2.8 A crystal structure of visual arrestin: a model for arrestin's regulation.

G protein-coupled signaling is utilized by a wide variety of eukaryotes for communicating information from the extracellular environment. Signal termination is achieved by the action of the arrestins, which bind to activated, phosphorylated G protein-coupled receptors. We describe here crystallographic studies of visual arrestin in its basal conformation. The salient features of the structure are a bipartite molecule with an unusual polar core. This core is stabilized in part by an extended carboxy-terminal tail that locks the molecule into an inactive state. In addition, arrestin is found to be a dimer of two asymmetric molecules, suggesting an intrinsic conformational plasticity. In conjunction with biochemical and mutagenesis data, we propose a molecular mechanism by which arrestin is activated for receptor binding.

Crystal structure of a eukaryotic initiator tRNA

OUR understanding of the molecular structure–function relationship in tRNA rests mainly on three types of information. First, on the common sequence patterns which have emerged from careful examination of many primary structures1–3; second, a wide variety of spectral and other physical and chemical results must be accounted for by the molecular structure4–6; and third, there is the detailed image of the yeast tRNAPhe molecule independently determined and refined from two different—albeit similar—crystal forms7–10. It is also clear, however, that the molecular model deduced from the yeast tRNAphe crystal structure cannot be easily reconciled with all structural requirements for function and is best considered a well-defined and stable canonical form of tRNA which is packed in an unusually well-ordered way in specific crystal lattices. Notwithstanding the enormous value of this canonical form in explaining the basic architectural features of tRNA, it is clearly important to image other crystalline tRNAs; particularly tRNAs that exhibit different functions (such as, initiators) or have significantly different covalent structures (for example, class III tRNAs)1 or those that crystallise in different solvent conditions. We report here the initial results of the crystal structure determination of a eukaryotic initiator tRNA crystallised from a highly polar aqueous solvent11,12. Its architecture is essentially the same as crystalline yeast tRNAphe, except for a small but significant difference in the position of the anticodon arm.