Two classes of alamethicin transmembrane channels: molecular models from single-channel properties

Molecular structures of transmembrane channels formed by alamethicin polypeptide aggregates were analyzed by measuring open-channel conductances and state-transition kinetics using voltage-clamp technique with artificial phospholipid bilayers isolated onto micropipettes by a novel solvent-free tip-dip method. Two distinct classes of alamethicin channels, each with a unique set of conductance states and kinetic properties, were identified. Alamethicin Rf50 at low temperatures forms mostly nonpersistent channels with lifetimes of < 1 min. Long-lasting persistent channels are formed by alamethicin Rf30 at all temperatures and by alamethicin Rf50 at room temperature. In the "modified barrel-stave" model for persistent channels based on the crystalline alamethicin secondary structure, the aqueous pore of the channel surrounded by parallel alamethicin monomers has a constriction generated by amino acid side chains protruding from the alamethicin helices into the pore. The model explains quantitatively the nonohmic channel conductance at high applied voltages and the conductance values and ion selectivities of various persistent channel states. The kinetic properties of nonpersistent channels are explained qualitatively by the "reversed-molecule" model in which nonpersistent channels differ from persistent channels by having one of the channel-forming alamethicin monomers oriented antiparallel to the others.