Reversal of charge selectivity in transmembrane protein pores by using noncovalent molecular adapters

In this study, the charge selectivity of staphylococcal alpha-hemolysin (al pha HL), a bacterial pore-forming toxin, is manipulated by using cyclodextr ins as noncovalent molecular adapters. Anion-selective versions of alpha HL , including the wild-type pore and various mutants, become more anion selec tive when beta-cyclodextrin (beta CD) is lodged within the channel lumen. B y contrast, the negatively charged adapter, hepta-6-sulfato-beta-cyclodextr in (s(7)beta CD), produces cation selectivity. The cyclodextrin adapters ha ve similar effects when placed in cation-selective mutant alpha HL pores. M ost probably, hydrated Cl- ions partition into the central cavity of beta C D more readily than K+ ions, whereas s(7)beta CD introduces a charged ring near the midpoint of the channel lumen and confers cation selectivity throu gh electrostatic interactions. The molecular adapters generate permeability ratios (PK+/PCl-) over a 200-fold range and should be useful in the de nov o design of membrane channels both for basic studies of ion permeation and for applications in biotechnology.