Glutamate efflux mediated by Corynebacterium glutamicum MscCG, Escherichia coli MscS, and their derivatives

Corynebacterium glutamicum is used in microbial biotechnology for the production of amino acids, in particular glutamate. The mechanism of glutamate excretion, however, is not yet fully understood. Recently, evidence was provided that the NCgl1221 gene product from C. glutamicum ATCC 13869, a MscS-type mechanosensitive efflux channel, is responsible for glutamate efflux [1]. The major difference of NCgl1221 and the homologous protein MscCG of C. glutamicum ATCC 13032 from Escherichia coli MscS and most other MscS-type proteins is the presence of an additional, 247 amino acid long C-terminal domain. By topology analysis, we show that this domain in MscCG carries a transmembrane segment. We have generated selected C-terminal truncations of MscCG, gain-of-function and loss-of-function constructs of both E. coli MscS and C. glutamicum MscCG, as well as fusion constructs of the two proteins. These mutant proteins were investigated for mechanosensitive efflux, MS channel activity, glutamate excretion and their impact on membrane potential. We provide evidence that the channel domain of MscCG mediates glutamate efflux in response to penicillin treatment, and that the E. coli MscS channel is to some extent able to function in a similar manner. We further show that the C-terminal domain of MscCG has a significant impact for function and/or regulation of MscCG. Significantly, a positive effect on glutamate efflux of the C-terminal extension of MscCG from C. glutamicum was also observed when fused to the E. coli MscS channel.