The Oligomeric States of the Purified Sigma 1 Receptor are Stabilized by Ligands [Signal Transduction]

May 30th, 2014 by Gromek, K. A., Suchy, F. P., Meddaugh, H. R., Wrobel, R. L., LaPointe, L. M., Chu, U. B., Primm, J. G., Ruoho, A. E., Senes, A., Fox, B. G.

Sigma 1 receptor (S1R) is a mammalian member of the ERG2 and sigma1 receptor like protein family (pfam04622). It has been implicated in drug addiction and many human neurological disorders including Alzheimers and Parkinsons diseases and amyotrophic lateral sclerosis. A broad range of synthetic small molecules including cocaine, (+)-pentazocine, haloperidol and small endogenous molecules such as N,N-dimethyltryptamine, sphingosine and steroids have been identified as regulators of S1R. However, the mechanism of activation of S1R remains obscure. Here we provide evidence in vitro that S1R has ligand binding activity only in an oligomeric state. The oligomeric state is prone to decay into an apparent monomeric form when exposed to elevated temperature, with loss of ligand binding activity. This decay is suppressed in the presence of the known S1R ligands such as haloperidol, BD-1047 and sphingosine. S1R has a GxxxG motif in its second transmembrane region, and these motifs are often involved in oligomerization of membrane proteins. Disrupting mutations within the GxxxG motif shifted the fraction of the higher oligomeric states towards smaller states and resulted in a significant decrease in specific [3H]-(+)-pentazocine binding. Results presented here support the proposal that S1R function may be regulated by its oligomeric state. Possible mechanisms of molecular regulation of interacting protein partners by S1R in the presence of small molecule ligands are discussed.