Sigma receptor photolabeling and sigma receptor-mediated modulation of potassium channels in tumor cells

Recent work has indicated that sigma receptor ligands can modulate potassiu m channels. However, the only sigma receptor characterized at the molecular level has a novel structure unlike any other receptor known to modulate io n channels. This 26-kDa protein has a hydropathy profile suggestive of a si ngle membrane-spanning domain, with no apparent regions capable of G-protei n activation or protein phosphorylation, In the present study patch clamp t echniques and photoaffinity labeling were used in DMS-114 cells (a tumor ce ll 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 tr ansduction, The sigma receptor Ligands N-allylnormetazocine (SKF10047), dit olyl-guanidine, and (+/-)-2-(N-phenylethyl-N-propyl)-amino-5-hy- droxytetra lin all inhibited voltage-activated potassium current (I-K). Iodoazidococai ne (IAC), a high affinity sigma receptor photoprobe, produced a similar inh ibition 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. P hotolabeling of this protein by IAC was inhibited by SKF10047 with half-max imal effect at 7 mu M. SKF10047 also inhibited I-K with a similar EC50 (14 mu M). 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 in ternally with either guanosine 5'-O-(2-thiodiphosphate) or GTP, indicating that signal transduction is independent of G-proteins, These results demons trate that ion channels can be modulated by a receptor that does not have s even membrane-spanning domains and does not employ G-proteins, Sigma recept ors thus modulate ion channels by a novel transduction mechanism.