Characterization of a photoalkylated psoralen receptor in HeLa cells.

Psoralens in combination with ultraviolet light are potent modulators of epidermal cell growth and differentiation. Responsive cell types contain specific, saturable, high-affinity binding sites for the psoralens. These binding sites become covalently modified by the psoralen molecule following ultraviolet light exposure. In the present studies the psoralen receptor, labeled with [3H]8-methoxypsoralen, was visualized in the cytoplasmic and plasma membrane fractions of HeLa cells following sodium dodecyl sulfate-polyacrylamide gel electrophoresis. The receptor had an apparent molecular mass of approximately 22,000 daltons and was shown to be sensitive to protease, but not nuclease treatment. The radiolabeled receptor could not be visualized in nuclear extracts of cells. Covalent binding of the radioligand to the receptor protein was inhibited by excess unlabeled 8-methoxypsoralen, indicating that covalent psoralen-receptor binding was saturable. In addition, the covalently modified receptor was found to persist in cells for over 5 h. The presence of a cellular protein that exhibits specific affinity for the psoralens and becomes photoalkylated by these compounds, together with previous data showing that the psoralens have direct effects on the cell surface membranes, supports our model that some of the biological effects of photoactivated psoralens are receptor-mediated.