The C-terminal α-helix is crucial for the activity of the bacterial ABC transporter BmrA.

ABC transporters are membrane integral proteins that consist of a transmembrane (TMD) and nucleotide-binding domain (NBD). Two monomers (half-transporters) of the Bacillus subtilis ABC transporter BmrA (Bacillus multidrug-resistance ATP) dimerize to build a functional full-transporter. As all ABC exporters, BmrA uses the free energy of ATP hydrolysis to transport substrate molecules across the cell membrane. For substrate transport, a BmrA dimer undergoes major conformational changes. ATP binding drives dimerization of the NBDs followed by the hydrolysis of the nucleotides. Conserved structural elements within the NBD and TMD are crucial for dimerization and the activity of BmrA. In the BmrA structure, an α-helix is present at the C-terminus, which can be subdivided in two smaller helices. As shown here, the very C-terminal helix (fragment) is not crucial for the BmrA activity. In fact, based on Cys-scanning mutagenesis, this region is highly flexible. In contrast, a BmrA variant lacking the entire C-terminal α-helix, showed no ATPase and transport activity. Via Ala-scanning we identified residues in the N-terminal fragment of the helix that are crucial for the BmrA activity, most likely via establishing contacts to structural elements involved in ATP recognition, binding and/or hydrolysis.