The fungal protein ERG2, a C-8 sterol isomerase, falls into the same protein family as sigma-1. Both localize to the ER membrane, although sigma-1 is also reported to be a cell surface receptor. Sigma-2 is an EXPREA domain protein[4] with a mostly intracellular (ER membrane) localization.[5]
^Skuza G, Rogóz Z (June 2006). "The synergistic effect of selective sigma receptor agonists and uncompetitive NMDA receptor antagonists in the forced swim test in rats". Journal of Physiology and Pharmacology. 57 (2): 217–29. PMID16845227.