Escherichia coli (bacteria)
We often think of bacteria as organism that cause diseases and in many cases that is true. This is however not entirely correct. In the body we find many bacteria that provide useful services. Escherichia coli (E.coli) is a bacterium that is very common in the human digestive tract. It usually does not cause any harm and E.coli is used as a source to create drugs for human use through genetic engineering. Some strains of E.coli however may cause disease. The most prominent might well be O157:H7, causing diarrhea and infections. The cause of these diseases is a toxin. It belongs to a family of proteins that is also found in an organism responsible for dysentery. They are the Shiga toxin family, named after a toxin from Shigella dysenteriae.
The structure shown here is part of such a toxin, produced by E.coli. This protein is structurally related to other toxins like cholera toxin or diphtheria toxin, having an A-B type structure. They consist of one or more B type subunits that bind to cell surfaces and one A type subunit that, when inserted into the cell, inhibits DNA synthesis. Diphtheria toxin for instance is an AB toxin and cholera toxin and the protein seen here are AB5 toxins. AB5 toxins have five copies of the B type subunit arranged as a pentamer. The structure here shows the B5 part of the protein. Each of the B units has a simple structure with a helix at the inside of the pentamer and a beta-sheet forming the external face of the pentamer. It is thought that the place where cell surface receptors bind to the B5 pentamer is a cleft formed by the contact of neighboring subunits. Thus there are five such potential binding site. It is hoped that by analyzing the structure of this cleft one may find clues for the design of effective drugs.
Protein Data Bank (PDB)
Stein, P.E. Boodhoo, A. Tyrrell, G.J. Brunton, J.L. Read, R.J.; "Crystal structure of the cell-binding B oligomer of verotoxin-1 from E. coli."; Nature; (1992) 355:748-750 PubMed:1741063.
author: Arno Paehler