Complete cysteine-scanning mutagenesis of the Salmonella typhimurium melibiose permease.

The melibiose permease of Salmonella typhimurium (MelB) catalyzes the stoichiometric symport of galactopyranoside with a cation (H, Li, or Na) and is a prototype for Na-coupled major facilitator superfamily (MFS) transporters presenting from bacteria to mammals. X-ray crystal structures of MelB have revealed the molecular recognition mechanism for sugar binding; however, understanding of the cation site and symport mechanism is still vague. To further investigate the transport mechanism and conformational dynamics of MelB, we generated a complete single-Cys library containing 476 unique mutants by placing a Cys at each position on a functional Cys-less background.