1lts
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(New page: 200px<br /><applet load="1lts" size="450" color="white" frame="true" align="right" spinBox="true" caption="1lts, resolution 1.95Å" /> '''REFINED STRUCTURE OF...)
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Revision as of 18:45, 20 November 2007
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REFINED STRUCTURE OF E. COLI HEAT LABILE ENTEROTOXIN, A CLOSE RELATIVE OF CHOLERA TOXIN
Overview
Heat-labile enterotoxin (LT) from Escherichia coli is a bacterial protein, toxin with an AB5 multimer structure, in which the B pentamer has a, membrane binding function and the A subunit is needed for enzymatic, activity. The LT crystal structure has been solved using a combination of, multiple isomorphous replacement, fivefold averaging and molecular, dynamics refinement. Phase combination using all these sources of phase, information was of crucial importance for the chain tracing. The structure, has now been refined to 1.95 A resolution, resulting in a model containing, 6035 protein atoms and 293 solvent molecules with a crystallographic, R-factor of 18.2% and good stereochemistry. The B subunits are arranged as, a highly stable pentamer with a donut shape. Each subunit takes part in, approximately 30 inter-subunit hydrogen bonds and six salt bridges with, its two neighbors, whilst burying a large surface area. The A subunit has, higher temperature factors and less well-defined secondary structure than, the B subunits. It interacts with the B pentamer mainly via the C-terminal, A2 fragment, which runs through the highly charged central pore of the B, subunits. The pore contains at least 66 water molecules, which fill the, space left by the A2 fragment. A detailed analysis of the contacts between, A and B subunits showed that most specific contacts occur at the entrance, of the central pore of the B pentamer, while the contacts within the pore, are mainly hydrophobic and water mediated, with the exception of two salt, bridges. Only a few contacts exist between the A1 fragment and the B, pentamer, showing that the A2 fragment functions as a "linker" of the A, and B parts of the protein. Interacting with the A subunit by the B, subunits does not cause large deviations from a common B subunit, structure, and the 5-fold symmetry is well maintained. A potential, NAD(+)-binding site is located in an elongated crevice at the interface of, two small sheets in the A1 fragment. At the back of this crevice the, functionally important Arg7 makes a hydrogen bond connecting two strands, which seems to be conserved across the ADP-ribosylating toxin family. The, putative catalytic residue (A1:Glu112) is located nearby, close to a very, hydrophobic region, which packs two loops together. This hydrophobic, region may be important for catalysis and membrane translocation.
About this Structure
1LTS is a Protein complex structure of sequences from Escherichia coli. Full crystallographic information is available from OCA.
Reference
Refined structure of Escherichia coli heat-labile enterotoxin, a close relative of cholera toxin., Sixma TK, Kalk KH, van Zanten BA, Dauter Z, Kingma J, Witholt B, Hol WG, J Mol Biol. 1993 Apr 5;230(3):890-918. PMID:8478941
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