The colicin la receptor, Cir, is also the translocator for colicin la

摘要

Colicin la, 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 la 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 la. We show that if the receptor-binding domain of colicin la 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 la using one Cir as its primary receptor (BtuB in the chimera) and a second Cir as the translocatlon pathway for its N-termlnal translocatlon (T) domain and its channel-forming C-terminal domain. Deletion of colicin la'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 la and protects E. coli from being killed by it. Thus, in addition to binding to colicin la's receptorbinding domain, Cir also binds weakly to its translocation domain.