Crystal Structure of Colicin Ia. Robert Stroud, Michael Wiener, Douglas Freymann, Partho Ghosh, Dept. of Biochemistry & Biophysics, UCSF, San Francisco, CA 94143-0448

The crystal structure of colicin Ia reveals a 210 Å long molecule with four distinct functional domains. A 100 amino acid sequence at the bend of the hairpin-like structure mediates binding to an outer-membrane receptor. Two 160 Å long a-helices link the receptor binding domain to the other domains, allowing these to span the periplasmic space. A three helix N-terminal domain mediates translocation across the outer membrane, and a 10 a-helix C-terminal domain forms an ion conducting channel in the plasma membrane of the target cell.