Gating the bacterial mechanosensitive channel MscL in vivo (original) (raw)

2002, Proceedings of the National Academy of Sciences

YggB and MscL are the major mechanosensitive channels in Escherichia coli, and each can rescue the double knockout mutant from osmotic downshock. However, the role of MscL in wild-type bacteria is in question, not only because cells without MscL survive severe osmotic downshocks, but because 1.8 times more suction is required to gate MscL than YggB under patch clamp. Here, we extend previous evidence [Ajouz, B., Berrier, C., Garrigues, A., Besnard, M. & Ghazi, A. (1998) J. Biol. Chem. 273, 26670-26674] to show that downshock gates MscL in vivo even in the presence of YggB. We have made this determination by engineering a channel we can structurally modify in vivo (Leu-193 Cys MscL). MscLs with charges in their constrictions are known to open easily and transiently to substates and stop cell growth. In this study, we use downshock to stretch this region open to allow attachment of a charged thiosulfonate reagent MTSET ؉ , thereby creating a toxic channel. Therefore, channel opening can be monitored by loss of colony forming units. By this measure, we find that an Ϸ800 mmol͞kg downshock from 1,200 mmol͞kg medium opens Leu-193 Cys MscL in the presence of YggB, but a downshock of only Ϸ400 mmol͞kg is required in the absence of YggB. In parallel, Leu-193 Cys MscL, stretched open by large sustained suction in the presence of MTSET ؉ in voltage-clamped patches, subsequently flickers open with little suction. These observations show that MscL opening is triggered by a specific downshock, even in the presence of YggB, that YggB buffers MscL gating in vivo, and that residue 19 becomes exposed upon channel opening.