Escherichia coli (bacteria)
Cell membranes are generally phospholipid bilayers with hydrophilic heads on the surfaces of the bilayer and the hydrophobic hydrocarbon chains forming the core of this layer. Since the core of the bilayer is hydrophobic, getting in and out of cells is thus no easy task for small hydrophilic molecules. One of the ways to shuttle ions across the cell membrane is by means of channels. These are holes in the membrane created by the interior of membrane bound proteins. Ion channels are one example of such proteins, porins are another example.
The structure here shows a bacterial porin. This protein is embedded in the cell membrane. While many membrane proteins typically contain many parallel helices oriented in the direction of the hydrocarbon chains, the topology in this protein is quite different. It has the shape of a barrel created by an 18-strand anti-parallel beta sheet. The barrel is about 50 angstrom long and between 30 to 40 angstrom in outer diameter. Two strands of the sheet take a detour to form a loop protruding into the interior of the barrel thus narrowing its inner diameter. Nevertheless the inside hole is still 10 angstrom at its narrowest cross section, sufficient to let small molecules of up to about 50 atoms total pass through. The pore is shaped like an hourglass: wider at the top and bottom and most narrow at its center. Thus molecules may more readily diffuse into the pore initially and which molecules may pass through, will be decided at the narrowest point both by size and character of the molecule. Differing amino acid residues at this point may give individual porins selectivity for certain types of molecules.
Protein Data Bank (PDB)
Cowan, S.W. Schirmer, T. Rummel, G. Steiert, M. Ghosh, R. Pauptit, R.A. Jansonius, J.N. Rosenbusch, J.P.; "Crystal structures explain functional properties of two E. coli porins."; Nature; (1992) 358:727-733 PubMed:1380671.
author: Arno Paehler