Expression, characterization, and purification of C-terminally hexahistidine-tagged thromboxane A2 receptors

Thromboxane A(2) (TxA(2)) receptors belong to the class of G-protein-coupled receptors. Knowledge of the relationship of structure to function for TxA(2) receptors is limited because of their low levels of expression, lengthy purification procedures and poor recoveries. A C-terminal hexahistidine-tag (C-His) was ligated to the a-isoform of TxA(2), receptors and expressed in COS-7 and Chinese hamster ovary cells. The C-His-TxA(2), receptors bound the radioligands I-125-7-[(1R,2S,3S,5R)-6,6-dimethyl-3-(4-benzenesulfonylamino)bicyclo[3.1.1]hept-2-y]-5(Z)-heptenoic acid, an antagonist, and I-125-[1S-1 alpha,2 beta-(5Z),3 alpha(1E,3S*),4 alpha]-7-[3[(3-hydroxy-4-(4'-phenoxy)-1-butenyl)-7-oxabicyclo-[2.2.1]heptan-2-yl]-heptanoic acid, an agonist, with affinities not significantly different from those of the wild type (wt)-TxA(2), receptors. LipofectAMINE transfection of the cDNAs resulted in high levels of expression (B-max = 95 +/- 6 pmol/mg) of the C-His-TxA(2), receptors. In competition binding studies the IC50 values of five different ligands were not significantly different between C-His-TxA(2), and wt-TxA(2), receptors. Agonist-induced stimulation of cAMP and total inositol phosphate formation were not significantly different between the two receptors. Purification on a Ni2+-NTA column resulted in a rapid (within 4 h) purification with a 36 +/- 2% recovery and a 30 +/- 6-fold purification (n = 5). The partially purified receptors were resolved on SDS-polyacrylamide gel electrophoresis, transferred to a nitrocellulose membrane, dissolved in acetone/trifluoroacetic acid/hexafluoroisopropanol/sinapinic acid, and successfully subjected to matrix-assisted laser desorption ionization-time of flight mass spectrometry analysis. The results suggest that the combination of a high level of expression of C-His-TxA(2), receptors and a rapid purification procedure followed by SDS- polyacrylamide gel electrophoresis may provide a useful approach for mass-spectrometry based structure-function and other studies of TxA(2), receptors.