Colicins are plasmid-encoded proteins which are synthesized by Escherichia coli and kill sensitive strains of E. coli and closely related species. Colicin B belongs to the group of channel-forming colicins (33), which include colicins A, E1, and I (11, 22, 31, 34, 35, 39). These transmembrane toxins depolarize the cytoplasmic membrane, leading to dissipation of cellular energy. Colicin B is usually encoded on large self-transmissible plasmids (15, 38), frequently in proximity to colicin M (30), It consists of a single polypeptide chain (6) with a molecular weight of about 60,000 (33).Comparison of the channels formed by colicins A and B in lipid bilayers under identical conditions showed a similar conductance characteristic. However, the ion flux through the colicin B channels was larger than through the A channels. In addition, the colicin B cha,nnels were more stable and less dependent on added CaCl2 than the colicin A channels (33). Crystals of the channel-forming domain of colicin'A have been obtained (40)

The pore-forming colicin B is taken up into Escherichia coli by a receptor and TonB-dependent process. The receptor and colicin B both contain a similar amino acid sequence, close to the N-terminal end, termed the TonB box. Point mutations were introduced into the TonB-box region of the colicin B structural gene cba resulting in colicin B derivatives which were partially or totally inactive against E. coli cells. All derivatives still bound to the receptor. An inactive derivative killed cells when translocated across the outer membrane by osmotic shock treatment, and formed pores in planar lipid bilayer membranes identical to the wild-type colicin. Some of the mutations were partially suppressed by mutations in the tonB structural gene. It was concluded that the TonB-box mutations define a region that is involved in the uptake of colicin B across the outer membrane.

Elevated putaminal dopamine turnover in de novo PD is associated with an increased risk for later motor complications and comprises a disease-intrinsic predisposing factor for their development.