The coat protein of bacteriophage Pf3 forms discrete and stable ion channels of uniform size in planar bilayers of asolectin. Its primary sequence suggests a channel formed by a bundle of transmembrane helixes. Since the apparent transmembrane region only consists of strongly hydrophobic residues, it represents a new class of channel-forming proteins. The channel activity is strongly voltage-dependent. The single-channel conductance of 60 pS (at 100 mV) in 0.2 M NaCl is slightly voltage-dependent, indicating conformational changes of the pore upon variation of the transmembrane elec. field. The channel is unselective which suggests that the pore is of aq. character. For the obsd. conductance, a channel diam. of 3.6 .ANG. is consistent with a tetrameric a-helix bundle, as calcd. from a barrel-stave model. A pronounced dependence of the gating kinetics with increasing voltage arises from two opposing effects: an increase in the no. of open channel structures, and a simultaneous, more than 3-fold decrease in the channel lifetime. Thus, a max. activity is reach around 100 mV, a range which corresponds well with physiol. membrane potentials. The channels activate only upon application of a pos. voltage on the side of the membrane to which the protein had been added. The slow relaxation of the mean current upon application of sudden voltage jumps indicates a strong activation barrier in the channel gating process, which may result from the membrane translocation of the charged residues of the peptide ends. A channel-mediated import mechanism is suggested for the bacterial infection by phage DNA. [on SciFinder (R)]