The Photoactivatable Inhibitor 7-azido-8-iodoketanserin Labels the N Terminus of the Vesicular Monoamine Transporter from Bovine Chromaffin Granules

In monoaminergic cells, the hormone or neurotransmitter is concentrated into secretory vesicles by a tetrabenazine- and reserpine-sensitive vesicular monoamine transporter (VMAT), catalyzing a H+/monoamine antiport. Ketanserin is another powerful inhibitor of VMAT that binds to the tetrabenazine binding site. A photoactivatable derivative, 7-azido-8-iodoketanserin (AZIK), labels covalently the transporter from bovine chromaffin granules, VMAT-2. Digestion with endoproteinases V8 or Lys-C, which cleave peptide bonds at acidic or lysine residues, respectively, revealed that the AZIK label is located in a 7 kDa segment of the VMAT-2 polypeptide. The photolabeled chromaffin granule transporter was purified by DEAE and WGA chromatography followed by selective aggregation and size-exclusion HPLC. After treatment by V8 or Lys-C, digestion products were separated by electrophoresis in SDS and sequenced. For both enzymes, the material comigrating with the labeled peptide produced a sequence matching the N terminus of VMAT-2. A K55E mutant of the bovine VMAT-2 cDNA was constructed and expressed in COS-7 cells. The mutant protein exhibited a full VMAT activity and could be labeled by AZIK. However, the formation of the 7 kDa labeled peptide upon Lys-C proteolysis was prevented in the mutant, with a redistribution of the label in higher-molecular mass digestion products. The localization of the label upstream of lysine 55 was confirmed by an immunological and enzymatic analysis. We conclude that the segment 2-55 of bovine VMAT-2, which encompasses the cytosolic N terminus and the first transmembrane segment in the current topological model of the transporter, contains residues involved in the binding of ketanserin and, possibly, tetrabenazine.