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Biochim Biophys Acta Biomembr. 2017 Jan;1859(1):48-60. doi: 10.1016/j.bbamem.2016.10.009. Epub 2016 Oct 23.

Interactions and cooperativity between P-glycoprotein structural domains determined by thermal unfolding provides insights into its solution structure and function.

Author information

1
Center for Structural Biology, University of Alabama at Birmingham, Birmingham, AL, USA.
2
Department of Cell Biology and Biochemistry, and Center for Membrane Protein Research, Texas Tech University Health Sciences Center, Lubbock, TX, USA.
3
Department of Cell Biology and Biochemistry, and Center for Membrane Protein Research, Texas Tech University Health Sciences Center, Lubbock, TX, USA. Electronic address: ina.urbatsch@ttuhsc.edu.
4
Center for Structural Biology, University of Alabama at Birmingham, Birmingham, AL, USA; Department of Chemistry, University of Alabama at Birmingham, Birmingham, AL, USA. Electronic address: christie@uab.edu.

Abstract

Structural changes in mouse P-glycoprotein (Pgp) induced by thermal unfolding were studied by differential scanning calorimetry (DSC), circular dichroism and fluorescence spectroscopy to gain insight into the solution conformation(s) of this ABC transporter that may not be apparent from current crystal structures. DSC of reconstituted Pgp showed two thermal unfolding transitions in the absence of MgATP, suggesting that each transition involved the cooperative unfolding of two or more interacting structural domains. A low calorimetric unfolding enthalpy and minimal structural changes were observed, which are hallmarks of the thermal unfolding of α-helical membrane proteins, because generally only the extramembranous regions undergo significant unfolding. Nucleotide binding increased the unfolding temperature of both transitions to the same extent, suggesting that one nucleotide binding domain (NBD) unfolds with each transition. Combined with the results from the two isolated NBDs, we propose that each DSC transition represents the cooperative unfolding of one NBD and the two contacting intracellular loops. Further, the presence of two transitions in both apo and MgATP bound wild-type Pgp suggests the NBD-dimeric conformation is transient, and that Pgp resides predominantly in the crystallographically observed inward-facing conformation with NBDs separated, even under conditions supporting continuous MgATP hydrolysis. In contrast, DSC of the vanadate-trapped MgADP·Pgp complex and the MgATP-bound catalytically inactive mutant, E552A/E1197A, show an additional transition at much higher temperature, corresponding to the unfolding of the nucleotide-trapped NBD-dimeric outward-facing conformation. The collective results indicate a strong preference for an NBD dissociated, inward-facing conformation of Pgp.

KEYWORDS:

ABC transporter; Circular dichroism; Detergent; Differential scanning calorimetry; Intrinsic fluorescence; P-glycoprotein

PMID:
27783926
DOI:
10.1016/j.bbamem.2016.10.009
[Indexed for MEDLINE]
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