Single channel behavior of matrix porin of Escherichia coli.

Porins are trimeric proteins in the outer membranes of Gram-negative bacteria. Several of them, among them matrix porin of Escherichia coli, form symmetrically voltage-gated ion channels in planar bilayers. Trimers exhibit negative resistance at potentials larger than +/- 90mV. Here we show that, after two pores within a trimer close irreversibly, the remaining third pore shows channel properties distinct from those observed in the trimer. This residual pore exhibits an asymmetric current-voltage dependence with a pronounced polarity-dependent shift toward low potentials and rates of channel-closing and opening that are one to two orders of magnitude faster than those observed for single channels in a reversibly voltage-dependent trimer. Rectification of single channels thus resembles that of a voltage-gated channel type observed in outer membrane patches of E.coli spheroblasts, hinting at the relevance of the phenomenon in vivo.