Evolutionary Specialization of Corynebacterium glutamicum MscCG, an Mscs-Like Mechanosensitive Channel, in Glutamate Export

Abstract

MscCG, mechanosensitive channel of Corynebacterium glutamicum, has been identified as the major glutamate exporter in this bacterium. This ion channel-based secretion system is used for industrial monosodium glutamate (MSG) production. Despite their unique structural and functional features, MscCG channels are gated by membrane tension according to the “Force-From-Lipids” principle, which applies to all known MscS-like mechanosensitive channels. Here we show that MscCG is an evolutionarily specialized mechanosensitive channel for glutamate export in C. glutamicum. Phylogenetic analysis suggested that two different C. glutamicum mechanosensitive channels, MscCG and MscCG2, are derived from one of the MscS paralogs by gene horizontal transfer rather than from canonical MscS. By using patch-clamp technique to record channel activities from C. glutamicum giant spheroplasts, we identified three types of endogenous mechanosensitive channels, MscCG, MscCG2, and CgMscL. The channels differ from each other in conductance and activation threshold, when negative pressure (suction) is applied to the spheroplast patch membrane. MscCG has a small conductance of ∼320 pS followed by an intermediate MscCG2 conductance of ∼1.0 nS and comparably large conductance of 3.5 nS exhibited by CgMscL. By applying Laplace's law, we determined that moderate membrane tension of ∼5.5 mN/m was required for half activation of MscCG compared to ∼12 mN/m required for half activation of both MscCG2 and CgMscL. Mechanical properties of the C. glutamicum spheroplast membrane were evaluated by micro-pipette aspiration method. The membrane was found to be very soft compared to the E. coli spheroplast membrane, which appears to have co-evolved with MscCG to fit the channel specialization in transport of amino acids, such as glutamate.