Transporters mediate the uptake and efflux of extremely diverse solutes including cations, anions, drugs, sugars, amino acids, neurotransmitters and metabolic end products (Pao et al.
). The nucleobase:cation symporter 1 (NCS-1) family is a family of electrochemical potential-driven transporters (Busch & Saier, 2002
). The NCS-1 family includes several hundred proteins found in bacteria, archaea, yeast, fungi and plants. The members of this family are typically composed of 400–650 amino acids and possess 12 putative transmembrane α-helices. These proteins transport small molecules such as thiamine, uracil, cytosine, purines, allantoin and nicotinamide riboside. Some of the NCS-1 family members function in uptake by substrate:H+
symport. At present, no structures of NCS-1 family members have been reported.
Recently, the membrane protein Mhp1, encoded by the hyuP
gene from Microbacterium liquefaciens
, has been cloned and expressed in Escherichia coli
(Suzuki & Henderson, 2006
). Mhp1 is composed of 489 amino acids with a molecular weight of 54.6 kDa and is predicted to have 12 transmembrane helices. Sequence analysis suggested that Mhp1 belongs to the NCS-1 family. Mhp1 transports 5-substituted hydantoin compounds, which are converted to amino acids as part of a metabolic salvage pathway. The transporter activity is proposed to be proton-dependent (Suzuki & Henderson, 2006
) because the uptake of a hydantoin compound was not accelerated by the addition of NaCl but was affected by pH. However, the details of the transporter mechanism of Mhp1 are unclear owing to a lack of structural information.
Here, we report the crystallization of Mhp1 from M. liquefaciens. The crystals diffracted to 2.85 Å resolution. Determination of the three-dimensional structure of Mhp1 will provide new insights into its function and more generally into the structure and function of NCS-1-family proteins.