Abstract
After the discovery of Corynebacterium glutamicum from avian feces-contaminated soil, its enigmatic L-glutamate secretion by corynebacterial MscCG-type mechanosensitive channels has been utilized for industrial monosodium glutamate production. Bacterial mechanosensitive channels are activated directly by increased membrane tension upon hypoosmotic downshock; thus; the physiological significance of the corynebacterial L-glutamate secretion has been considered as adjusting turgor pressure by releasing cytoplasmic solutes. In this review, we present information that corynebacterial mechanosensitive channels have been evolutionally specialized as carriers to secrete L-glutamate into the surrounding environment in their habitats rather than osmotic safety valves. The lipid modulation activation of MscCG channels in L-glutamate production can be explained by the “Force-From-Lipids” and “Force-From-Tethers” mechanosensing paradigms and differs significantly from mechanical activation upon hypoosmotic shock. The review also provides information on the search for evidence that C. glutamicum was originally a gut bacterium in the avian host with the aim of understanding the physiological roles of corynebacterial mechanosensing. C. glutamicum is able to secrete L-glutamate by mechanosensitive channels in the gut microbiota and help the host brain function via the microbiota–gut–brain axis.
Highlights
Corynebacterial Mechanosensitive Channel Model for the Industrial L-Glutamate ProductionThe Corynebacterium glutamicum ATCC13032 strain was first isolated as a glutamate producer in the avian feces-contaminated soil in 1957 by Kinoshita andUdaka [1], who described the result of an elaborated screening method with a glutamateauxotrophic bacterium Leuconostoc mesenteroides strain, P-60 [2]
Liposomes made of the negatively charged lipids, DOPG, were shown to have softer mechanical properties than liposomes made of the neutral lipids DOPC by atomic force microscopy [86]. These findings indicate that corynebacterial membranes are much more deformable by mechanical stimuli than other bacterial membranes, and, that membrane tension, increased by largely expanded membranes upon osmotic downshock, will activate C. glutamicum mechanosensitive channels, MscCG, MscCG2 and CgMscL in the cytoplasmic membranes
These findings indicate that corynebacterial membranes are much more deformable by mechanical stimuli than other bacterial membranes, and, that membrane tension, increased by largely expanded membranes 8upon of 19 osmotic downshock, will activate C. glutamicum mechanosensitive channels, MscCG, MscCG2 and CgMscL in the cytoplasmic membranes
Summary
Corynebacterial Mechanosensitive Channel Model for the Industrial L-Glutamate Production. Using a novel patch-clamp technique with C. glutamicum giant spheroplasts of the industrial strain ATCC13869, all endogenous mechanosensitive channels including MscCG and MscCG2 were recorded electrophysiologically in the native membrane environment [27] These studies strongly supported the “mechanosensitive channel model” [28] and explained the L-glutamate secretion mechanisms as follows: (1) specific treatments alter membrane tension by inhibiting membrane lipids or cell wall synthesis;. The mechanosensitive solute efflux system releases betaine or proline, Microorganisms 2021, 9, x FOR PEER REVIEW preferably to other amino acids of similar size, and does not excrete ATP in C. glutamicum [42] This is higher substance specificity than the counterpart of the E. coli system that is non-selective releases even small proteins. The cytoplasmic betaine concentration is fine-tuned by the leakage through the mechanosensitive channel MscCG as the pump and leak mechanism
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