Structure and molecular mechanism of an anion-selective mechanosensitive channel of small conductance

Abstract

Mechanosensitive (MS) channels are universal cellular membrane pores. Bacterial MS channels, as typified by MS channel of small conductance (MscS) from Escherichia coli (EcMscS), release osmolytes under hypoosmotic conditions. MS channels are known to be ion selective to different extents, but the underlying mechanism remains poorly understood. Here we identify an anion-selective MscS channel from Thermoanaerobacter tengcongensis (TtMscS). The structure of TtMscS closely resembles that of EcMscS, but it lacks the large cytoplasmic equatorial portals found in EcMscS. In contrast, the cytoplasmic pore formed by the C-terminal β-barrel of TtMscS is larger than that of EcMscS and has a strikingly different pattern of electrostatic surface potential. Swapping the β-barrel region between TtMscS and EcMscS partially switches the ion selectivity. Our study defines the role of the β-barrel in the ion selection of an anion-selective MscS channel and provides a structural basis for understanding the ion selectivity of MscS channels.crystal structure | anion selection | cytoplasmic region | electrophysiology | single channel recording

Selectivity mechanism of the mechanosensitive channel MscS revealed by probing channel subconducting states

“…The cytoplasmic domain is also considered to have a role in the adaptation and inactivation of the channel 25,26,29 and has been suggested to serve as a sensor for cytoplasmic crowding 19,30 . Furthermore, it has been proposed that the MscS cytoplasmic vestibulum may have the role of an ion-selectivity filter in this channel 6,20,31,32 .…”

Bacterial Mechanosensitive Channels: Models for Studying Mechanosensory Transduction

“…Unlike MscL, which has no preference for any ions, MscS exhibits a weak preference for anions compared with cations (43,95,140). This weak ion selectivity has recently been shown to originate from the charged residues within seven vestibular portals in the MscS cytoplasmic chamber, which is interesting, because, unlike voltage-gated K + , Na + and Ca 2 + channels, the selectivity of MscS is not determined by charged residues in the channel pore but by residues outside the pore in the waterfilled cytoplasmic domain (36,47,170).…”

Recent characterizations of MscS and its homologs provide insight into the basis of ion selectivity in mechanosensitive channels

“…1). 19 Analysis of EcMscS/TtMscS chimeras provides evidence that the difference in ion selectivities can be attributed in part, to this β-barrel sequence (which is distinct from the middle β-domain, indicated in blue). Elaborations to this C-terminal β-barrel sequence may in fact underlie the ability to transport specialized solutes such as glutamate.…”

“…We previously demonstrated that EcMscS can be expressed mechanosensitive channel pore through the binding of ions, small molecules or other proteins. 11,14 Several MscS homologs have been characterized by single-channel electrophysiology including MscMJ and MscMJLR from Methanococcus jannaschii, 15 MscK and MscM from E. coli, [16][17][18] TtMscS from Thermoanaerobacter tengcongensis, 19 MscCG from Corynebacterium glutamicum 20 and MSC1, a chloroplasttargeted homolog from the green alga Chlamydomonas reinhardtii. 21 In addition, a family of bacterial cyclic nucleotide gated channels with homology to MscS has been reported.…”

“…This is similar to the ion selectivity of MSC1 from Chlamydomonas (P Cl :P K = 7) 21 and that of TtMscS (P Cl :P K = 8.7). 19 Other MscS homologs either show a slight preference for anions (P Cl :P K = 1.2 -3 for EcMscS) [4][5][6][7] or demonstrate a preference for cations (MscMJ and MscMJLR have P Cl :P K = 0.17 and 0.2 respectively, 15 MscM has P Cl :P K = 0.4 17,18 and MscCG has P Cl :P K = 0.3 20 ). These preferences are moderate; for comparison, the bacterial potassium channel KcsA prefers K + over Na + by a factor of over 1,000.…”