Membrane protein structures as ribbon diagrams. (Left) An outer membrane β-barrel, OmpF (Cowan, S.W., T. Schirmer, G. Rümmel, M. Steient, R. Gosh, R.A. Pauptit, J.N. Jansonius, and J.P. Rosenbusch. 1992. Nature. 358:727), PDB accession code 2OMF, resolution 2.4 Å. Right: a plasma membrane α-helical bundle, KcsA (Doyle, D.A., J.M. Cabral, R.A. Pfuetzner, A. Kuo, J.M. Gulbis, S.L. Cohen, B.T. Chait, and R. MacKinnon. 1998. Science. 280:69), PDB accession code 1Bl8, resolution 3.2 Å. We thank Dr. C. Lima (Weill Medical College of Cornell University, New York, NY) for assistance with the figure, which was produced using the program SETOR (Evans, S. 1993. J. Mol. Graph. 11:134).