title: Dual incorporation of photoaffinity ligands on dopamine transporters implicates proximity of labeled domains.
abstract: We have recently developed novel high-affinity blockers for the dopamine transporter (DAT) by carrying out structure-activity studies of GBR 12909 molecule piperidine analogs. To investigate the molecular basis of binding of these compounds in comparison to known sites of action of GBR 12909, cocaine, and benztropine analogs, we developed a piperidine-based photoaffinity label [(125)I]4-[2-(diphenylmethoxy)ethyl]-1-[(4-azido- 3-iodophenyl)methyl]-piperidine [(125)I]AD-96-129), and used proteolysis and epitope-specific immunoprecipitation to identify the protein domains that interact with the ligand. [(125)I]AD-96-129 became incorporated into two different regions of the DAT primary sequence, an N-terminal site containing transmembrane domains (TMs) 1 to 2, and a second site containing TMs 4 to 6. Both of these regions have been identified previously as sites involved in the binding of other DAT photoaffinity labels. However, in contrast to the previously characterized ligands that showed nearly complete specificity in their binding site incorporation, [(125)I]AD-96-129 became incorporated into both sites at comparable levels. These results suggest that the two domains may be in close three-dimensional proximity and contribute to binding of multiple uptake blockers. We also found that DATs labeled with [(125)I]AD-96-129 or other photoaffinity labels displayed distinctive sensitivities to proteolysis of a site in the second extracellular loop, with protease resistance related to the extent of ligand incorporation in the TM4 to 6 region. These differences in protease sensitivity may indicate the relative proximity of the ligands to the protease site or reflect antagonist-induced conformational changes in the loop related to transport inhibition.
authors: Department of Biochemistry and Molecular Biology, University of North Dakota School of Medicine and Health Sciences, Grand Forks, North Dakota, USA. rvaughan@medicine.nodak.edu
authors: R A
authors: RA
authors: Vaughan
authors: J D
authors: JD
authors: Gaffaney
authors: J R
authors: JR
authors: Lever
authors: M E
authors: ME
authors: Reith
authors: A K
authors: AK
authors: Dutta
mesh_terms: D000818:Animals; D001386:Azides / Q000378:metabolism; D002352:Carrier Proteins / Q000737:chemistry / Q000276:immunology / Q000378:metabolism*; D003042:Cocaine / Q000031:analogs & derivatives / Q000378:metabolism; D003342:Corpus Striatum / Q000378:metabolism; D050483:Dopamine Plasma Membrane Transport Proteins; D010450:Endopeptidases / Q000378:metabolism; D000939:Epitopes / Q000276:immunology; D066298:In Vitro Techniques; D007457:Iodine Radioisotopes; D008024:Ligands; D008562:Membrane Glycoproteins; D026901:Membrane Transport Proteins; D009419:Nerve Tissue Proteins; D010449:Peptide Mapping; D019920:Photoaffinity Labels / Q000378:metabolism*; D010880:Piperidines / Q000378:metabolism; D011233:Precipitin Tests; D051381:Rats
chemical_list: C423824:4-(2-(diphenylmethoxy)ethyl)-1-benzyl piperidine; D001386:Azides; D002352:Carrier Proteins; D050483:Dopamine Plasma Membrane Transport Proteins; D000939:Epitopes; D007457:Iodine Radioisotopes; D008024:Ligands; D008562:Membrane Glycoproteins; D026901:Membrane Transport Proteins; D009419:Nerve Tissue Proteins; D019920:Photoaffinity Labels; D010880:Piperidines; C493216:Slc6a3 protein, rat; C076379:RTI 82; D010450:Endopeptidases; D003042:Cocaine