Amino Acid Exporters in Corynebacterium glutamicum

Abstract

Corynebacterium glutamicum was isolated by Japanese researchers in 1957 as an L-glutamic acid-producing bacterium. L-Glutamate has a distinctive taste known as ‘umami’, and is widely used as a flavour enhancer. Since 1960, L-glutamate has been industrially produced by fermentation using C. glutamicum. C. glutamicum is also used for the fermentative production of other amino acids such as L-lysine and L-threonine. In the past decade, our understanding of the molecular basis of amino acid secretion has enabled the identification of lysE, thrE, and brnFE, i.e., the genes encoding novel carriers exporting L-lysine, L-threonine, and L-isoleucine, respectively. Although it has been suggested that the secretion of L-glutamate by C. glutamicum is mediated by a carrier system in the cytoplasmic membrane, no L-glutamate exporter has been identified for a long time. In 2007, it was reported that NCgl1221 gene, which encodes a homolog of the MscS protein (mechanosensitive channel of the small conductance), is involved in L-glutamate secretion by this bacterium. It was later designated as MscCG (mechanosensitive channel from C . glutamicum). Recently, it was also reported that most of C. glutamicum strains have one more MscS homolog named MscCG2, which functions as a minor L-glutamate exporter. These two MscS homologs were proved to function as a mechanosensitive channel by electrophysiological studies. Although the physiological significance of amino acid export systems in bacterial cells remains unclear, understanding the export processes of amino acids is important for amino acid production. The focus of this review is on the amino acid export systems of C. glutamicum, especially on the recently identified mechanosensitive channels NCgl1221/MscCG and MscCG2.