Conductance and ion selectivity of a mesoscopic protein nanopore probed with cysteine scanning mutagenesis.

Nanometer-scale proteinaceous pores are the basis of ion and macromolecular transport in cells and organelles. Recent studies suggest that ion channels and synthetic nanopores may prove useful in biotechnological applications. To better understand the structure-function relationship of nanopores, we are studying the ion-conducting properties of channels formed by wild-type and genetically engineered versions of Staphylococcus aureus alpha-hemolysin (alphaHL) reconstituted into planar lipid bilayer membranes. Specifically, we measured the ion selectivities and current-voltage relationships of channels formed with 24 different alphaHL point cysteine mutants before and after derivatizing the cysteines with positively and negatively charged sulfhydryl-specific reagents. Novel negative charges convert the selectivity of the channel from weakly anionic to strongly cationic, and new positive charges increase the anionic selectivity. However, the extent of these changes depends on the channel radius at the position of the novel charge (predominantly affects ion selectivity) or on the location of these charges along the longitudinal axis of the channel (mainly alters the conductance-voltage curve). The results suggest that the net charge of the pore wall is responsible for cation-anion selectivity of the alphaHL channel and that the charge at the pore entrances is the main factor that determines the shape of the conductance-voltage curves.