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TRANSMEMBRANE HELIX 12
VGIILTLAMNIMSTLQWAVNSI
(amino acids 1129 thru 1150)
Mutations in this helix have been shown to change the block of CFTR pore by diphenyl-2-carboxylic acid and may therefore form part of the pore.
TM6 and TM12 are the two most conserved helices, in terms of sequence, when comparing CFTRs from different species. TM1 thru TM6 tend to be more conserved than TM7 thru TM12.
S1141 apparently binds the channel blocker DPC when the serine at 341 in helix 6 is mutated out. And when T1134 is changed to a phenylalanine, the affinity for DPC is increased even further. McDonough et al concluded on this basis that both helix 6 and helix 12 line the pore. For information on pore, see helix1.
In June, 2001, Gupta et al. (from Dalhousie University, Halifax, Nova
Scotia) investigated the role of the 12th transmembrane helix because of it's
positional homology to the 6th transmembrane helix. They
mutated five residues in potentially important regions of the twelfth
transmembrane region individually to alanines. They then checked the
function of the mutant channels using patch clamp recording after they expressed
them in mammalian cell lines. They concluded that "...three of the
five mutations significantly weakened block of unitary Cl(-) currents by SCN(-),
implying a partial disruption of anion binding within the pore. Two of
these mutations also caused a large reduction in the steady-state channel mean
open probability, suggesting a role for the twelfth transmembrane region in
channel gating." These mutations had no effect on anion
selectivity or unitary chloride conductance of CFTR, however, as would have been
predicted based on similar mutations in transmembrane helix 6.
They therefore concluded that "... the relatively minor effects of these
five mutations on channel permeation properties suggests that, despite their
symmetrical positions within the CFTR protein, the sixth and twelfth
transmembrane regions make highly asymmetric contributions to the functional
properties of the pore." Gupta J, Evagelidis A,
Hanrahan JW, Linsdell P. Biochemistry 2001 Jun 5;40(22):6620-7