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Function

Mhp1 is a transmembrane protein bellowing to the nucleobase-cation-symport-1 (NCS1) transporter family from Microbacterium liquefaciens. It allows the sodium dependent income of indolyl methyl- and benzyl-hydantoins in the cell. Those are part of a salvage metabolic pathway leading to their conversion in amino acids.

Figure 1 : Structure of benzyl-hydantoin

Disease

Relevance

Structural highlights

Global structure

The protein is composed of one chain of 12 transmembrane alpha helices (TMs). They are organised in two repeating units connected by a 59-residue loop (TMs 1-5 and TMs 6-10) and two additional helices (TM 11 and 12). The two repeating units have a symmetrical topology (Figure 2).

Figure 2 : Structure of Mhp1 from Microbacterum liquefaciens

The central bundle is composed of TMs 1 and 2, twined to the TMs 6 and 7 respectively. In addition, the protein presents a V-shape structure formed by TMs 3 to 5, twined to TMs 8 to 10 (Figure 2.A).

The substrate- and cation-binding sites are located in the space between the central four-helix-bundle and the outer helix layer.

Structure of the substrate biding site

The substrate binding site is located at the break of the TMs 1 and 6. The benzyl-hydantoin interacts with the amino acids of the binding site. The hydantoin group establisches pi-stacking interactions with the indole ring of Trp 117 and Trp 220 and hydrogen bounds with Asn 318 and Gln 121. The benzyl ring interacts with Trp 220 and Gln 42 (Figure 3).

Figure 3 : Substrate binding site

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