Expression and localization of the Corynebacterium glutamicum NCgl1221 protein encoding an L-glutamic acid exporter

Corynebacterium glutamicum strains are used for the fermentative production of L-glutamate. Biotin limitation and addition of fatty acid ester surfactants can induce L-glutamic acid secretion. However, the mechanism of L-glutamate secretion remains unclear. It was recently reported that the NCgl1221 protein, a mechanosensitive channel homolog, was considered to be functional as an important L-glutamate exporter. However, the structure of the NCgl1221 protein has not been studied recently. In this study, we predicted the topology structure of the NCgl1221 protein by TopPred. We further analyzed the expression and localization of the NCgl1221 protein in the cytoplasmic membrane of both Escherichia coli and C. glutamicum cells by fusing green fluorescence protein (GFP) in the C terminus of the NCgl1221 protein. We found that the expressed fusion protein NCgl1221/GFP was visualized in the periphery of both E. coli and C. glutamicum cells under confocal microscope, which is consistent with a cytoplasmic membrane location. In contrast, GFP was ubiquitous in the cytoplasm of bacterial cells expressing GFP only. We herein provide the straightforward and obvious evidence to prove that NCgl1221 is confined to the cytoplasmic membrane. And NCgl1221 is a membrane protein having four transmembrane segments with its C terminus in the cytoplasm.