3.A.1.24.1
The L- and D-methionine porter (also transports formyl-L-methionine and other methionine derivatives) (Zhang et al., 2003). The 3.7A structure of MetNI has been solved. An allosteric regulatory mechanism operates at the level of transport activity, so increased intracellular levels of the transported ligand stabilize an inward-facing, ATPase-inactive state of MetNI to inhibit further ligand translocation into the cell (Kadaba et al., 2008).  The structure of an MetQ homologue in Neisseria meningitidis has been solved at 2.25 Å resolution revealing a bound methionine in the cleft between the two domains (Yang et al. 2009). Conformational changes in MetQ provide substrate access through the binding protein to the transmembrane translocation pathway. MetQ likely mediates uptake of methionine derivatives through two mechanisms: in the methionine-bound form, substrate is delivered from the periplasm to the transporter (the canonical mechanism) and in the apo form, it facilitates ligand binding when complexed to the transporter (the noncanonical mechanism). This dual role of substrate-binding proteins was proposed to provide a kinetic strategy for ABC transporters to transport both high- and low-affinity substrates present in a physiological concentration range (Nguyen et al. 2018).