Reversal of Charge Selectivity in Transmembrane Protein Pores by Using Noncovalent Molecular Adapters

In this study, the charge selectivity of staphylococcal α -hemolysin(α HL), a bacterial pore-forming toxin, is manipulated by using cyclodextrins as noncovalent molecular adapters. Anion-selective versions of α HL, including the wild-type pore and various mutants, become more anion selective when β -cyclodextrin(β CD) is lodged within the channel lumen. By contrast, the negatively charged adapter, hepta-6-sulfato-β -cyclodextrin (s7β CD), produces cation selectivity. The cyclodextrin adapters have similar effects when placed in cation-selective mutant α HL pores. Most probably, hydrated Cl-ions partition into the central cavity of β CD more readily than K+ions, whereas s7β CD introduces a charged ring near the midpoint of the channel lumen and confers cation selectivity through electrostatic interactions. The molecular adapters generate permeability ratios (PK+ /PCl- ) over a 200-fold range and should be useful in the de novo design of membrane channels both for basic studies of ion permeation and for applications in biotechnology.