Biomedical Engineering Reference
In-Depth Information
X
OUT
CYTOSOL
TMDs
C
N
NB1
NB2
ICDs
(a)
NBDs
TM1-12
OUT
CYTOSOL
NBD1
NBD2
(b)
(c)
FIGURE 10.1.
Topology and structure of Pgp. (
a
) Pgp is proposed to consist of two equivalent
halves, each with six TM segments and an NB domain on the cytosolic side. Both the N- and
C-terminus are cytosolic. (
b
) The low-resolution structural model of Pgp generated using
several different FRET measurements of the distances separating key regions of the protein.
(
c
) Medium-resolution structural model of Pgp obtained from cryo-EM studies. Top: a side
view of the protein is shown with the NB domains at the bottom. The 12 putative TM
α
-helices
are arranged in a pseudosymmetrical relationship. Bottom: view of Pgp looking down on the
TM helices from the extracellular side of the membrane. The dashed lines indicate the putative
boundary of a 4.5-nm-thick lipid bilayer (scale bar
=
5 nm), [(
b
) From ref. 158; (
c
) adapted
from ref. 45, with permission.]
In addition, Loo et al. showed that Cys residues in the Walker A motifs could be
cross-linked at low temperatures to Cys residues in the LSGGQ motif, indicating
that the signature sequences in one NB domain are adjacent to the Walker A site
in the other NB domain.
41
Later work by Rosenberg and co-workers showed that
nucleotide binding causes a repacking of the TM regions of Pgp,
42
which could open
the central pore to allow access of hydrophobic drugs directly from the lipid bilayer.
43
It was proposed from this reorganization that ATP binding, not hydrolysis, drives the
conformational changes associated with transport.
42
The vanadate-trapped complex of
Pgp, which is thought to resemble the catalytic transition state structurally, displayed
a third distinct conformation of the protein, suggesting that rotation of TM
α
-helices
had taken place.
42
Mouse Pgp has also been studied by EM and image analysis
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