The SecY complex forms a channel capable of ionic discrimination

Protein translocation across the bacterial membrane occurs at the SecY complex or channel. The resting SecY channel is impermeable to small molecules owing to a plug domain that creates a seal. Here, we report that a channel loosely sealed, or with a plug locked open, does not, however, lead to general membrane permeability. Instead, strong selectivity towards small monovalent anions, especially chloride, is observed. Mutations in the pore ring-structure increase both the translocation activity of the channel and its ionic conductance, however the selectivity is maintained. The same ionic specificity also occurs at the onset of protein translocation and across the archaeal SecY complex. Thus, the ion-conducting characteristic of the channel seems to be conserved as a normal consequence of protein translocation. We propose that the pore ring-structure forms a selectivity filter, allowing cells to tolerate channels with imperfect plugs.