Karan Kapoor, Shashank Pant, and Emad Tajkhorshid.
Active participation of membrane lipids in inhibition of multidrug
transporter P-glycoprotein.
Chemical Science, 12:6293-6306, 2021.
KAPO2021-ET
P-glycoprotein (Pgp) is a major efflux pump in humans, overexpressed in a variety of cancers
and associated with the development of multi-drug resistance.
Allosteric modulation by various ligands (e.g., transport substrates, inhibitors, and ATP)
has been biochemically shown to directly influence structural dynamics, and thereby, the function of
Pgp.
However, the molecular details of such effects, particularly with respect to the role and involvement
of the surrounding
lipids, are not well established.
Here, we employ all-atom molecular dynamics (MD) simulations
to study the conformational landscape of Pgp
in the presence of a high-affinity, third-generation inhibitor, tariquidar,
in comparison to the nucleotide-free (APO) and the ATP-bound states, in order
to characterize the mechanical effects of the inhibitor that might be of relevance to its blocking
mechanism of Pgp.
Simulations in a multi-component lipid bilayer show a dynamic equilibrium between open(er) and more
closed inward-
facing (IF) conformations in the APO state, with binding of ATP shifting the equilibrium towards
conformations more
prone to ATP hydrolysis and subsequent events in the transport cycle.
In the presence of the inhibitor bound to the drug-binding pocket within the transmembrane domain
(TMD),
Pgp samples more open IF conformations, and
the nucleotide binding domains (NBDs) become highly dynamic.
Interestingly, and reproduced in multiple independent simulations,
the inhibitor is observed to facilitate recruitment of lipid molecules into the Pgp lumen through the
two proposed drug-
entry portals,
where the lipid head groups from the cytoplasmic leaflet penetrate into and, in some cases,
translocate inside the TMD,
while the lipid tails remain extended into the bulk lipid environment.
These ``wedge" lipids likely enhance the inhibitor-induced conformational restriction of the TMD
leading to the
differential modulation of coupling pathways observed with the NBDs downstream.
We suggest a novel inhibitory mechanism for tariquidar, and potentially for related third-generation
Pgp inhibitors, where
lipids are seen to enhance the inhibitory role in the catalytic cycle of membrane transporters.
