J Biol Chem, Vol. 274, Issue 26, 18387-18392, June 25, 1999

Sigma Receptor Photolabeling and Sigma Receptor-mediated Modulation of Potassium Channels in Tumor Cells

Russell A. Wilke, Rakesh P. Mehta, Patrick J. Lupardus, Yuenmu Chen^**, Arnold E. Ruoho^**, and Meyer B. Jackson

From the Departments of  Medicine,  Physiology, and ^** Pharmacology, University of Wisconsin School of Medicine, Madison, Wisconsin 53706

Recent work has indicated that sigma receptor ligands can modulate potassium channels. However, the only sigma receptor characterized at the molecular level has a novel structure unlike any other receptor known to modulate ion channels. This 26-kDa protein has a hydropathy profile suggestive of a single membrane-spanning domain, with no apparent regions capable of G-protein activation or protein phosphorylation. In the present study patch clamp techniques and photoaffinity labeling were used in DMS-114 cells (a tumor cell line known to express sigma receptors) to investigate the role of the 26-kDa protein in ion channel modulation and probe the mechanism of signal transduction. The sigma receptor ligands N-allylnormetazocine (SKF10047), ditolylguanidine, and (±)-2-(N-phenylethyl-N-propyl)-amino-5-hydroxytetralin all inhibited voltage-activated potassium current (I_K). Iodoazidococaine (IAC), a high affinity sigma receptor photoprobe, produced a similar inhibition in I_K, and when cell homogenates were illuminated in the presence of IAC, a protein with a molecular mass of 26 kDa was covalently labeled. Photolabeling of this protein by IAC was inhibited by SKF10047 with half-maximal effect at 7 µM. SKF10047 also inhibited I_K with a similar EC₅₀ (14 µM). Thus, physiological responses to sigma receptor ligands are mediated by a protein with the same molecular weight as the cloned sigma receptor. This indicates that ion channel modulation is indeed mediated by this novel protein. Physiological responses were the same when cells were perfused internally with either guanosine 5'-O-(2-thiodiphosphate) or GTP, indicating that signal transduction is independent of G-proteins. These results demonstrate that ion channels can be modulated by a receptor that does not have seven membrane-spanning domains and does not employ G-proteins. Sigma receptors thus modulate ion channels by a novel transduction mechanism.