The Nucleobase‐Cation‐Symport‐1 Family of Membrane Transport Proteins

Abstract The evolutionary relationships of membrane transport proteins of the nucleobase‐cation‐symport (NCS‐1) family from bacteria, fungi, and plants are described. The reported substrates of the NCS‐1 family include nucleobases, hydantoins, and vitamins. Secondary active transport of substrate accompanied by a sodium ion, or possibly a proton, is the usual mechanism of energization. A strategy for the amplified expression, purification, and activity assays of bacterial members of the NCS‐1 family is described. Conditions are given for the production of diffracting crystals of one member, the Na + ‐coupled transporter for aromatic hydantoins, ‘Mhp1’, from Microbacterium liquefaciens . The 3D structures of three forms of the Mhp1 protein are discussed in terms of one open‐outward, one substrate‐occluded, and one open‐inward conformation contributing to a molecular dynamics simulation of the alternating‐access model of membrane transport. The unexpected similarity of the protein fold of Mhp1 to those of transport proteins, hitherto thought to be from different evolutionary families, is discussed.