Crystallization and characterization of colicin E1 channel-forming polypeptides.

Crystals of the channel-forming domain of colicin E1 from E. coli were grown by vapor diffusion at pH 6.4 and higher pH values. Cleavage of the colicin molecule with trypsin or thermolysin produced two of the pore-forming polypeptides used in these experiments. The third polypeptide was purified from a constructed plasmid that overexpresses only the C-terminal domain of colicin E1. Polypeptide crystals are tetragonal with space group I4, have one monomer in the asymmetric unit, and diffract to 2.2-2.4 A. Unit cell parameters for the tryptic and thermolytic polypeptides are a = 102.9 A and c = 35.6 A. Crystals of the overexpressed polypeptide have unit cell parameters of a = 87.2 A and c = 59.1 A. The crystals were characterized by precession photography, and native data sets of each channel-forming fragment were collected on a Siemens-Nicolet area detector. The crystallization and characterization of these polypeptides are the first steps in the structure determination of the channel-forming domain of colicin E1.