TY - JOUR
T1 - P-glycoprotein is fully active after multiple tryptophan substitutions
AU - Swartz, Douglas J.
AU - Weber, Joachim
AU - Urbatsch, Ina L.
N1 - Funding Information:
We thank Guillermo Altenberg for careful reading of the manuscript, Roger B. Sutton for help with modeling a lipid POPC bilayer in the Pgp structure (shown in the Graphical abstract) using the program Orientation of Proteins in Membranes (OPM), and Ping Bai and Sri Karan Botta for excellent technical assistance. This work was supported by the Cancer Prevention and Research Institute of Texas RP101073 , the National Institute of Health RGM102928 and U54-GM94610 , and the South Plains Foundation .
PY - 2013/3
Y1 - 2013/3
N2 - P-glycoprotein (Pgp) is an important contributor to multidrug resistance of cancer. Pgp contains eleven native tryptophans (Trps) that are highly conserved among orthologs. We replaced each Trp by a conservative substitution to determine which Trps are important for function. Individual Trp mutants W44R, W208Y, W132Y, W704Y and W851Y, situated at the membrane surface, revealed significantly reduced Pgp induced drug resistance against one or more fungicides and/or reduced mating efficiencies in Saccharomyces cerevisiae. W158F and W799F, located in the intracellular coupling helices, abolished mating but retained resistance against most drugs. In contrast, W228F and W311Y, located within the membrane, W694L, at the cytoplasmic membrane interface, and W1104Y in NBD2 retained high levels of drug resistance and mating efficiencies similar to wild-type Pgp. Those were combined into pair (W228F/W311Y and W694L/W1104Y) and quadruple (W228F/W311Y/W694L/W1104Y) mutants that were fully active in yeast, and could be purified to homogeneity. Purified pair and quad mutants exhibited drug-stimulated ATPase activity with binding affinities very similar to wild-type Pgp. The combined mutations reduced Trp fluorescence by 35%, but drug induced fluorescence quenching was unchanged from wild-type Pgp suggesting that several membrane-bound Trps are sensitive to drug binding. Overall, we conclude that Trps at the membrane surface are critical for maintaining the integrity of the drug binding sites, while Trps in the coupling helices are important for proper interdomain communication. We also demonstrate that functional single Trp mutants can be combined to form a fully active Pgp that maintains drug polyspecificity, while significantly reducing intrinsic fluorescence.
AB - P-glycoprotein (Pgp) is an important contributor to multidrug resistance of cancer. Pgp contains eleven native tryptophans (Trps) that are highly conserved among orthologs. We replaced each Trp by a conservative substitution to determine which Trps are important for function. Individual Trp mutants W44R, W208Y, W132Y, W704Y and W851Y, situated at the membrane surface, revealed significantly reduced Pgp induced drug resistance against one or more fungicides and/or reduced mating efficiencies in Saccharomyces cerevisiae. W158F and W799F, located in the intracellular coupling helices, abolished mating but retained resistance against most drugs. In contrast, W228F and W311Y, located within the membrane, W694L, at the cytoplasmic membrane interface, and W1104Y in NBD2 retained high levels of drug resistance and mating efficiencies similar to wild-type Pgp. Those were combined into pair (W228F/W311Y and W694L/W1104Y) and quadruple (W228F/W311Y/W694L/W1104Y) mutants that were fully active in yeast, and could be purified to homogeneity. Purified pair and quad mutants exhibited drug-stimulated ATPase activity with binding affinities very similar to wild-type Pgp. The combined mutations reduced Trp fluorescence by 35%, but drug induced fluorescence quenching was unchanged from wild-type Pgp suggesting that several membrane-bound Trps are sensitive to drug binding. Overall, we conclude that Trps at the membrane surface are critical for maintaining the integrity of the drug binding sites, while Trps in the coupling helices are important for proper interdomain communication. We also demonstrate that functional single Trp mutants can be combined to form a fully active Pgp that maintains drug polyspecificity, while significantly reducing intrinsic fluorescence.
KW - Conservative tryptophan substitutions
KW - Drug binding sites
KW - Membrane bilayer
KW - Native tryptophans
KW - P-glycoprotein
KW - Polyspecificity
UR - http://www.scopus.com/inward/record.url?scp=84872144837&partnerID=8YFLogxK
U2 - 10.1016/j.bbamem.2012.12.005
DO - 10.1016/j.bbamem.2012.12.005
M3 - Article
C2 - 23261390
AN - SCOPUS:84872144837
SN - 0005-2736
VL - 1828
SP - 1159
EP - 1168
JO - Biochimica et Biophysica Acta - Biomembranes
JF - Biochimica et Biophysica Acta - Biomembranes
IS - 3
ER -