*2.2. Gln347, Gln<sup>725</sup> and Gln<sup>990</sup> Modulate the Transport of the Xanthene Dye Calcein-AM*

In order to test the importance of the hydrogen bonds formed between taxol and the side chains of ABCB1 amino acids Gln347, Gln725, and Gln990, the wild-type glutamines were replaced with alanine by site-directed mutagenesis. Alanine with its non-polar side chain is unable to donate an equivalent hydrogen bond to the transport substrate but its smaller side chain should be tolerated in the tertiary structure of the protein. Individual, pairwise mutants and a triple (Qtriple) mutant were generated.
