the invariant lysine residue in the Walker A motif of ABC transporters has been suggested to be crucial for the binding of the L- and Q- phosphates of the nucleotides while the conserved aspartate in site B is thought to play a role in liganding
the Mg2 ion that accompanies the nucleotide. Detailed mutational analyses performed with numerous ABC transporters con¢rmed that amino acid changes in the Walker A and B motifs are generally not tolerated with respect to ATPase activity