2.A.2.1.1
Melibiose permease. Catalyzes the coupled stoichiometric symport of a galactoside with a cation (either Na⁺, Li⁺, or H⁺). Based on LacY, a 3-d model has been derived (Yousef and Guan, 2009). Asp55 and Asp59 are essential for Na⁺ binding. Asp124 may play a critical role by allowing Na⁺-induced conformational changes and sugar binding. Asp19 may facilitate melibiose binding (Granell et al., 2010).  The alternate access mechanism fits better into a flexible gating mechanism rather than the archetypical helical rigid- body rocker-switch mechanism (Wang et al. 2016).  Crystal structures of Salmonella typhimurium MelB in two conformations, representing an outward partially occluded and an outward inactive state (Ethayathulla et al. 2014). MelB adopts a typical MFS fold and contains a previously unidentified cation-binding motif. Three conserved acidic residues form a pyramidal-shaped cation-binding site for Na⁺, Li⁺ or H⁺, which is in close proximity to the sugar-binding site. Both cosubstrate-binding sites are mainly contributed by the residues from the amino-terminal domain (Ethayathulla et al. 2014). The Glucose Enzyme IIA protein of the PTS binds MelB either in the absence or presence of a galactoside, and binding decreases the affinity for melibiose, giving rise to inducer exclusion (Saier 1989; Hariharan and Guan 2014). A D55C mutant converted MelB_St to a solely H⁺-coupled symporter, and together with the free-energy perturbation calculation, Asp59 is the sole protonation site of MelB_S of Salmonella typhimurium. Unexpectedly, the H⁺-coupled melibiose transport exhibited poor activities at greater bulky ΔpH and better activities at reversal ΔpH, supporting the novel theory of transmembrane-electrostatically localized protons and the associated membrane potential are the primary driving forces for H⁺-coupled symport mediated by MelB_St (Hariharan et al. 2024).  MelB_St trapped by camelid single-domain nanobodies (Nbs) retained its physiological functions, and the trapped conformation is similar to that bound by the physiological regulator EIIA^Glc (Katsube et al. 2023).