title: The colicin Ia receptor, Cir, is also the translocator for colicin Ia.
abstract: Colicin Ia, a channel-forming bactericidal protein, uses the outer membrane protein, Cir, as its primary receptor. To kill Escherichia coli, it must cross this membrane. The crystal structure of Ia receptor-binding domain bound to Cir, a 22-stranded plugged beta-barrel protein, suggests that the plug does not move. Therefore, another pathway is needed for the colicin to cross the outer membrane, but no 'second receptor' has ever been identified for TonB-dependent colicins, such as Ia. We show that if the receptor-binding domain of colicin Ia is replaced by that of colicin E3, this chimera effectively kills cells, provided they have the E3 receptor (BtuB), Cir, and TonB. This is consistent with wild-type Ia using one Cir as its primary receptor (BtuB in the chimera) and a second Cir as the translocation pathway for its N-terminal translocation (T) domain and its channel-forming C-terminal domain. Deletion of colicin Ia's receptor-binding domain results in a protein that kills E. coli, albeit less effectively, provided they have Cir and TonB. We show that purified T domain competes with Ia and protects E. coli from being killed by it. Thus, in addition to binding to colicin Ia's receptor-binding domain, Cir also binds weakly to its translocation domain.
authors.affiliation: Department of Physiology and Biophysics, Albert Einstein College of Medicine, 1300 Morris Park Avenue, Bronx, NY 10461, USA. karen.jakes@einstein.yu.edu
authors.forename: Karen S
authors.initials: KS
authors.lastname: Jakes
authors.forename: Alan
authors.initials: A
authors.lastname: Finkelstein
mesh_terms: D003087:Colicins / Q000737:chemistry / Q000378:metabolism*; D004926:Escherichia coli / Q000378:metabolism*; D029968:Escherichia coli Proteins / Q000737:chemistry / Q000378:metabolism*; D008565:Membrane Proteins / Q000378:metabolism; D011485:Protein Binding; D017434:Protein Structure, Tertiary; D021381:Protein Transport; D011956:Receptors, Cell Surface / Q000737:chemistry / Q000378:metabolism*
chemical_list: D003087:Colicins; D029968:Escherichia coli Proteins; D008565:Membrane Proteins; D011956:Receptors, Cell Surface; C028593:colicin receptor, E coli; C489181:tonB protein, E coli