Biomedical subjects

Michael D Purdy

Publications and source records attributed to Michael D Purdy.

3 recordsLinked to original sources

PubMed2006

Crystallization and preliminary X-ray crystallographic analysis of the Escherichia coli outer membrane cobalamin transporter BtuB in complex with the carboxy-terminal domain of TonB.

The energy-dependent uptake of organometallic compounds and other micronutrients across the outer membranes of Gram-negative bacteria is carried out by outer membrane active-transport proteins that utilize the proton-motive force of the inner membrane via coupling to the TonB protein. The Escherichia coli outer membrane cobalamin transporter BtuB and a carboxy-terminal domain of the TonB protein, residues 147-239 of the wild-type protein, were expressed and purified individually. A complex of BtuB and TonB(147-239) was formed in the presence of the substrate cyanocobalamin (CN-Cbl; vitamin B12) and calcium and was crystallized. BtuB was purified in the detergent LDAO (n-dodecyl-N,N-dimethylamine-N-oxide) and the complex was formed in a detergent mixture of LDAO and C8E4 (tetraethylene glycol monooctylether). Crystals were obtained by sitting-drop vapor diffusion, with the reservoir containing 30%(v/v) polyethylene glycol (PEG 300) and 100 mM sodium acetate pH 5.2. The crystals belong to space group P2(1)2(1)2(1) (unit-cell parameters a = 74.3, b = 82.4, c = 122.6 angstroms). The asymmetric unit consists of a single BtuB-TonB complex. Data sets have been collected to 2.1 angstroms resolution at a synchrotron beamline (APS SER-CAT 22-ID).

Bacterial Outer Membrane Proteins↗

PubMed2006

Outer membrane active transport: structure of the BtuB:TonB complex.

In Gram-negative bacteria, the import of essential micronutrients across the outer membrane requires a transporter, an electrochemical gradient of protons across the inner membrane, and an inner membrane protein complex (ExbB, ExbD, TonB) that couples the proton-motive force to the outer membrane transporter. The inner membrane protein TonB binds directly to a conserved region, called the Ton-box, of the transporter. We solved the structure of the cobalamin transporter BtuB in complex with the C-terminal domain of TonB. In contrast to its conformations in the absence of TonB, the Ton-box forms a beta strand that is recruited to the existing beta sheet of TonB, which is consistent with a mechanical pulling model of transport.

Bacterial Outer Membrane Proteins↗

PubMed2002

Thiol-reactive lanthanide chelates for phasing protein X-ray diffraction data.

Lanthanides can contribute a large anomalous component to X-ray scattering when present and ordered in a target crystal. This large anomalous signal is a useful source of phase information in X-ray crystallographic studies of biological macromolecules. Thiol-reactive lanthanide chelates were tested as a means of incorporation of lanthanides into protein crystals. Two compounds, each capable of being loaded with a lanthanide of choice, were synthesized: diethylenetriaminepentaacetic 3-(2-pyridyldithio)propionyl hydrazide (DTPA-PDPH) and 1,4,7,10-tetraazacyclododecane-N,N',N",N"'-tetraacetic 3-(2-pyridyldithio)propionyl hydrazide (DOTA-PDPH). A cysteine mutant of the 34 kDa phosphate-binding protein (PBP-A197C) from Escherichia coli was used as a test case. PBP-A197C was labeled with DTPA-PDPH loaded with dysprosium. Characteristics of DTPA-PDPH enabled spectroscopic monitoring of the labeling reaction. Complete labeling of PBP-A197C was confirmed by mass spectrometry and SDS-PAGE analysis. Labeled PBP-A197C (PBP-A197C-DTPA-Dy) crystallized identically to unlabeled protein. X-ray diffraction data were collected from PBP-A197C-DTPA-Dy crystals in-house with a Cu Kalpha rotating-anode source and with a tuneable synchrotron source (ALS 5.0.2). Synchrotron data were collected at energies corresponding to the Dy L(III) edge f" peak and a high-energy remote. Each data set was treated as an independent SAD experiment. A large anomalous signal was present in the data collected in-house and at the synchrotron. The Dy site was easily located in anomalous difference Patterson maps calculated from each of the data sets. In each case, SAD phasing resulted in high-quality electron-density maps, as evidenced by the success of automated model building. The generality of the method was analyzed with several other test proteins. Labeling of some of these proteins with thiol-reactive lanthanide chelates was deleterious to protein solubility or crystallization. In two of the cases the lanthanide chelate was disordered in the crystals. These results suggest that this method may not be well suited for high-throughput crystallography. However, for difficult cases requiring a large anomalous signal, thiol-reactive lanthanide chelates may prove to be a valuable tool.

Chelating Agents↗