Membrane-protein models representing the two general categories of transmembrane-region structure. (a) Side-view of BtuCD Vitamin B12 Transporter  containing an α-helical transmembrane region (PDB accession code 1L7V); and (b) side-view of FecA Ferric Citrate Uptake Receptor  featuring a β-barrel transmembrane region (PDB accession code 1KMO). Light gray blocks on the sides of each model depict the approximate limits of the lipid bilayer. The figure was produced using MOLSCRIPT  and Raster3D .
Walian et al. Genome Biology 2004 5:215