HUMAN PLATELET PLASMA-MEMBRANE - CHARACTERIZATION OF THE SEROTONIN TRANSPORTER

After treatment of human platelets by a sulfhydryl-dependent bacterial protein cytolysin, a glycoprotein was reproducibly purified by a one-step affinity chromatography using 6-fluorotryptamine as ligand and elution by serotonin (5-HT), cyanoimipramine, citalopram, or a Na+-free buffer. The purified fraction migrated on sodium dodecyl sulfate-polyacrylamide gel electrophoresis as a single band with an apparent molecular mass of 68 kDa. The purified glycoprotein bound the 5-HT uptake blockers H-3-paroxetine, H-3-cyanoimipramine, and H-3-citalopram with Kds similar to the ones observed for intact human platelets. No binding was detected with H-3-hydroxytetrabenazine, H-3-ouabain, H-3-gamma aminobutyric acid or H-3-BTCP, the respective markers of the granular monoamine transporter, the plasma membrane Na+, K+-ATPase, the gamma aminobutyric acid and dopamine carriers. The purified 68-kDa glycoprotein is therefore likely to correspond at least to the paroxetine and imipramine binding domains of the 5-HT transporter located at the human platelet plasma membrane. Finally a 68-kDa protein was purified in the same conditions from the human megakaryocytic cell line Dami and to a lesser extent from the human megakaryoblastic cell line MEG-01 but not from the human erythroleukaemic cell line HEL.