Methionine Uptake in Corynebacterium glutamicum by MetQNI and by MetPS, a Novel Methionine and Alanine Importer of the NSS Neurotransmitter Transporter Family

The soil bacterium Corynebacterium glutamicum is a model organism in amino acid biotechnology. Here we present the identification of two different L-methionine uptake systems including the first characterization of a bacterial secondary methionine carrier. The primary carrier MetQNI is a high affinity ABC-type transporter specific for L-methionine. Its expression is under the control of the transcription factor McbR, the global regulator of sulfur metabolism in C. glutamicum. Besides MetQNI a novel secondary methionine uptake system of the NSS (neurotransmitter: sodium symporter) family was identified and named MetP. The MetP system is characterized by a lower affinity for methionine and uses Na+ ions for energetic coupling. It is also the main alanine transporter in C. glutamicum and is expressed constitutively. These observations are consistent with models of methionine, alanine, and leucine bound to MetP, derived from the X-ray crystal structure of the LeuT transporter from Aquifex aeolicus. Complementation studies show that MetP consists of two components, a large subunit with 12 predicted transmembrane segments and, surprisingly, an additional subunit with one predicted transmembrane segment only. Thus, this new member of the NSS transporter family adds a novel feature to this class of carriers, namely, the functional dependence on an additional small subunit.