Expression and activity of the nucleotide-binding domains of the human ABCA1 transporter

The aim of this study was the expression and production in Escherichia coli of the nucleotide-binding domains (NBDs) of the human ABCA1 transporter, in a soluble, non-denatured form. To increase the protein solubility, and avoid expression in E. coli inclusion bodies, we extended the length of the expressed NBD domains, to include proximal domains. The corresponding cDNA constructs were used to express the N-terminal His-tagged WT and mutant proteins, which were purified by Ni(2+)-affinity chromatography. Optimal expression of soluble proteins was obtained for constructs including the NBD, the downstream 80-residue domain, and about 20 upstream residues. The size homogeneity of WT and mutant NBDs was determined by Dynamic Light Scattering, and ATP-binding constants and ATPase activities were measured. The NBD1 and NBD2 domains bound ATP with comparable affinity. The ATPase activity of WT His-NBD1 was about three times higher than that of NBD2 and amounted to 5913 compared to 1979 nmol Pi/micromol NBD/min for WT His-NBD2. All engineered mutants had comparable ATPase activity to the corresponding WT protein. The optimisation of the length of the expressed proteins, based upon the boundary prediction of NBDs and neighbour domains, enables the expression and purification of soluble ABCA1 NBDs, with high ATPase activity. This approach should prove useful for the study of the structural and functional properties of the NBDs and other domains of the ABC transporters.