THE 18 kDa SIGMA‐2 Receptor and PGRMC1 are Derived From Separate Genes

Abstract

The sigma‐2 receptor (S2R) is a unique binding site expressed in many mammalian tissues. Due to high expression in tumors, the S2R is a promising target for cancer diagnostic imaging. However, the sequence of the S2R has remained a matter of debate. Historically the identity of S2R has been determined by these criteria: (1) the S2R binds with high affinity to 1,3‐di‐o‐tolylguanidine (DTG) and haloperidol but not the selective sigma‐1 receptor ligand, (+)‐pentazocine; (2) specific photolabeling with [3H]Azido‐DTG, and [125I]‐iodoazido‐fenpropimorph ([125I]IAF) has shown the apparent molecular size of S2R to be 18‐21kDa. The progesterone receptor membrane component 1 (PGRMC1), a 25 kDa protein, has been recently reported to be the S2R. To establish the identity of the S2R, we created a PGRMC1 knockout NSC34 cell line using CRISPR/Cas9 technology. We found that in NSC34 cells devoid of PGRMC1, the maximum binding (Bmax) of haloperidol protectable [3H]‐DTG binding was similar to that of wild‐type control cells. Moreover, in both control and PGRMC1‐KO cells, [125I]IAF selectively photolabeled a DTG protectable S2R‐18kDa protein. A series of previously reported highly selective S2R compounds showed protection of [125I]‐IAF photolabeling of the S2R‐18kDa in PC12 cell membranes that contain both PGRMC1 and S2R‐18kDa. These results support the conclusion that PGRMC1 and S2R‐18kDa are separate proteins derived from two different genes. Cloning and characterization of the S2R‐18kDa will provide further clarification.