Aaron J. Oakley

The 2.2 A crystal structure of a pocilloporin pigment reveals a nonplanar chromophore conformation.

Reef-building corals contain host pigments, termed pocilloporins, that function to regulate the light environment of their resident microalgae by acting as a photoprotectant in excessive sunlight. We have determined the crystal structure of an intensely blue, nonfluorescent pocilloporin to 2.2 A resolution and a genetically engineered fluorescent variant to 2.4 A resolution. The pocilloporin chromophore structure adopts a markedly different conformation in comparison with the DsRed chromophore, despite the chromophore sequences (Gln-Tyr-Gly) being identical; the tyrosine ring of the pocilloporin chromophore is noncoplanar and in the trans configuration. Furthermore, the fluorescent variant adopted a noncoplanar chromophore conformation. The data presented here demonstrates that the conformation of the chromophore is highly dependent on its immediate environment.

The crystal structures of glutathione S-transferases isozymes 1-3 and 1-4 from Anopheles dirus species B.

Glutathione S‐transferases (GSTs) are dimeric proteins that play an important role in cellular detoxification. Four GSTs from the mosquito Anopheles dirus species B (Ad), an important malaria vector in South East Asia, are produced by alternate splicing of a single transcription product and were previously shown to have detoxifying activity towards pesticides such as DDT. We have determined the crystal structures for two of these alternatively spliced proteins, AdGST1–3 (complexed with glutathione) and AdGST1–4 (apo form), at 1.75 and 2.45 Å resolution, respectively. These GST isozymes show differences from the related GST from the Australian sheep blowfly Lucilia cuprina; in particular, the presence of a C‐terminal helix forming part of the active site. This helix causes the active site of the Anopheles GSTs to be enclosed. The glutathione‐binding helix α2 and flanking residues are disordered in the AdGST1–4 (apo) structure, yet ordered in the AdGST1–3 (GSH‐bound) structure, suggesting that insect GSTs operate with an induced fit mechanism similar to that found in the plant phi‐ and human pi‐class GSTs. Despite the high overall sequence identities, the active site residues of AdGST1–4 and AdGST1–3 have different conformations.

Identification, characterization and structure of a new Delta class glutathione transferase isoenzyme

The insect GST (glutathione transferase) supergene family encodes a varied group of proteins belonging to at least six individual classes. Interest in insect GSTs has focused on their role in conferring insecticide resistance. Previously from the mosquito malaria vector Anopheles dirus, two genes encoding five Delta class GSTs have been characterized for structural as well as enzyme activities. We have obtained a new Delta class GST gene and isoenzyme from A. dirus, which we name adGSTD5-5. The adGSTD5-5 isoenzyme was identified and was only detectably expressed in A. dirus adult females. A putative promoter analysis suggests that this GST has an involvement in oogenesis. The enzyme displayed little activity for classical GST substrates, although it possessed the greatest activity for DDT [1,1,1-trichloro-2,2-bis-(p-chlorophenyl)ethane] observed for Delta GSTs. However, GST activity was inhibited or enhanced in the presence of various fatty acids, suggesting that the enzyme may be modulated by fatty acids. We obtained a crystal structure for adGSTD5-5 and compared it with other Delta GSTs, which showed that adGSTD5-5 possesses an elongated and more polar active-site topology.

Flexibility revealed by the 1.85 Å crystal structure of the β sliding-clamp subunit of Escherichia coli DNA polymerase III

The beta subunit of the Escherichia coli replicative DNA polymerase III holoenzyme is the sliding clamp that interacts with the alpha (polymerase) subunit to maintain the high processivity of the enzyme. The beta protein is a ring-shaped dimer of 40.6 kDa subunits whose structure has previously been determined at a resolution of 2.5 A [Kong et al. (1992), Cell, 69, 425-437]. Here, the construction of a new plasmid that directs overproduction of beta to very high levels and a simple procedure for large-scale purification of the protein are described. Crystals grown under slightly modified conditions diffracted to beyond 1.9 A at 100 K at a synchrotron source. The structure of the beta dimer solved at 1.85 A resolution shows some differences from that reported previously. In particular, it was possible at this resolution to identify residues that differed in position between the two subunits in the unit cell; side chains of these and some other residues were found to occupy alternate conformations. This suggests that these residues are likely to be relatively mobile in solution. Some implications of this flexibility for the function of beta are discussed.

Characterization of a family 11 xylanase from Bacillus subtillis B230 used for paper bleaching.

Enzymes such as family 11 xylanases are increasingly being used for industrial applications. Here, the cloning, structure determination and temperature-stability data of a family 11 xylanase, Xyn11X, from the alkali-tolerant Bacillus subtilis subspecies B230 are reported. This enzyme, which degrades xylan polymers, is being produced on an industrial scale for use in the paper-bleaching industry. Xyn11X adopts the canonical family 11 xylanase fold. It has a greater abundance of side chain to side chain hydrogen bonds compared with all other family 11 xylanase crystal structures. Means by which the thermostability of Xyn11X might be improved are suggested.

Macromolecular crystallography as a tool for investigating drug, enzyme and receptor interactions.

1. Protein crystallography is an essential tool for the discovery and investigation of pharmacological interactions at the molecular level. It allows investigators to directly visualize the three‐dimensional structures of proteins, including enzymes, receptors and hormones.

Crystallization of two glutathione S-transferases from an unusual gene family

Two glutathione S-transferase isozymes from the mosquito Anopheles dirus (AdGST1-3 and AdGST1-4) from an alternately spliced gene family have been expressed, purified and crystallized. The isozymes share an N-terminal domain derived from a single exon and C-terminal domains from unique exons. Despite the high level of sequence identity (64% overall), the two isozymes crystallize in different space groups, the 1-3 isozyme in P3(1)21 or P3(2)21 (unit-cell parameters a = 49.9, c = 271.8 A at 100 K) and the 1-4 isozyme in P4(1) or P4(3) (unit-cell parameters a = 87.8, c = 166.1 at 100 K). Determination of these structures will advance our understanding of how these enzymes inactivate pesticides and the structural consequences of alternate splicing.