Leishmania major
aquaglyceroporin LmAQP1 allows adventitious passage of
antimonite, an activated form of the
drug Pentostam, which is used as the first line treatment for
leishmaniasis. The extracellular C-loop of an
aquaglyceroporin confers substrate specificity. Alteration of Glu125 to
serine in the Plasmodium falciparum
aquaglyceroporin PfAQP has been shown to selectively affect water but not
glycerol permeability. The C-loop of LmAQP1 is twelve residues longer than PfAQP, and Ala163 is at an equivalent position as Glu125 of PfAQP. The role of Ala163 in LmAQP1 solute permeability was investigated. Alteration of Ala163 to
serine or
threonine did not significantly affect conduction of solutes. However, alteration to
aspartate,
glutamate, and
glutamine blocked passage of water,
glycerol, and other organic solutes. While LmAQP1 is a
mercurial insensitive water channel, mutation of the adjacent
threonine (Thr164) to
cysteine led to inhibition of water passage by Hg(2+). This inhibition could be reversed upon addition of β-
mercaptoethanol. These data suggest that, unlike Glu125 (PfAQP), Ala163 is not involved in stabilization of the C-loop and selective solute permeability. Ala163 is located near the pore mouth of the channel, and replacement of Ala163 by bulkier residue sterically hinders the passage of solutes. Alteration of Ala163 to
serine or
threonine affected
metalloid uptake in the order, wild-type>A163S>A163T.
Metalloid conduction was near completely blocked when Ala163 was mutagenized to
aspartate,
glutamate, or
glutamine. Mutations such as A163S and A163T that reduced the permeability to
antimonite, without a significant loss in water or solute conductivity raises the possibility that, subtle changes in the side chain of the
amino acid residue in position 163 of LmAQP1 may play a role in drug resistance.