THE CRYSTAL STRUCTURE OF COLICIN IA. Michael Wiener, Douglas Freymann, Partho Ghosh, Robert Stroud. Department of Biochemistry & Biophysics, University of California, San Francisco, 94143-0448.

The structure of the entire channel-forming bacteriocin (protein toxin) colicin Ia has been solved to a resolution of 2.4Å by multiple isomorphous replacement. The formation of ion-permeable channels in target cell membranes is a general mechanism of cytotoxicity. The process involves secretion of a soluble protein which inserts into the plasma membrane of the target cell and forms a lethal pore. Colicins, Escherichia coli protein toxins, are a well-characterized example of this class of proteins. Colicin Ia crystals, comprised of approximately 80% solvent, are in spacegroup C222₁ (a=64.4Å, b=178.6Å, c=285.5Å). All data sets used in the structure determination were collected from frozen crystals with a synchrotron light source (SSRL beamline 7-1). Heavy atom derivatives were obtained using mercurial soaks of engineered single-site cysteine mutants.

The structure of the 69kD colicin Ia protein reveals the structural relationships between the three distinct domains which function, respectively, to i) bind to a receptor on the outer membrane of susceptible bacteria, ii) translocate across the outer membrane through the receptor, and iii) bind to the inner membrane and form a pore in the presence of the transmembrane voltage. The domains are separated by an extraordinarily long helical coiled-coil.