Biosynthesis of ionotropic acetylcholine receptors requires the evolutionarily conserved ER membrane complex

Abstract

The number of nicotinic acetylcholine receptors (AChRs) present in the plasma membrane of muscle and neuronal cells is limited by the assembly of individual subunits into mature pentameric receptors. This process is usually inefficient, and a large number of the synthesized subunits are degraded by endoplasmic reticulum (ER)-associated degradation. To identify cellular factors required for the synthesis of AChRs, we performed a genetic screen in the nematode Caenorhabditis elegans for mutants with decreased sensitivity to the cholinergic agonist levamisole. We isolated a partial loss-of-function allele of ER membrane protein complex-6 (emc-6), a previously uncharacterized gene in C. elegans. emc-6 encodes an evolutionarily conserved 111-aa protein with two predicted transmembrane domains. EMC-6 is ubiquitously expressed and localizes to the ER. Partial inhibition of EMC-6 caused decreased expression of heteromeric levamisole-sensitive AChRs by destabilizing unassembled subunits in the ER. Inhibition of emc-6 also reduced the expression of homomeric nicotine-sensitive AChRs and GABA A receptors in C. elegans muscle cells. emc-6 is orthologous to the yeast and human EMC6 genes that code for a component of the recently identified ER membrane complex (EMC). Our data suggest this complex is required for protein folding and is connected to ER-associated degradation. We demonstrated that inactivation of additional EMC members in C. elegans also impaired AChR synthesis and induced the unfolded protein response. These results suggest that the EMC is a component of the ER folding machinery. AChRs might provide a valuable proxy to decipher the function of the EMC further.

Identification of Zika Virus and Dengue Virus Dependency Factors using Functional Genomics

“…More recently, the EMC has been postulated to serve as a tethering mechanism that facilitates PL transfer from the ER to the mitochondria in yeast (Lahiri et al, 2014) and to mediate the folding of multi-pass, but not single-pass, proteins in the ER (Richard et al, 2013;Satoh et al, 2015). We became interested in the EMC because it was the top complex enriched for in all of our flavivirus MORR and CRISPR/Cas9 screens.…”

“…The EMC is conserved from yeast to humans, participates in ER protein folding response (with its loss eliciting the unfolded protein response (UPR) [Jonikas et al, 2009]), and is needed for ER-to-mitochondrial lipid trafficking (Lahiri et al, 2014) and the expression of several multi-pass transmembrane proteins, including rhodopsin, in the Drosophila eye (Satoh et al, 2015) and the acetylcholine receptor in C. elegans (Richard et al, 2013). EMC1-EMC6 were shown to form a complex after their identification in a screen in yeast for ER protein folding factors (Jonikas et al, 2009).…”

C. elegans Punctin Clusters GABAA Receptors via Neuroligin Binding and UNC-40/DCC Recruitment

“…LGI , unc-40(e1430), unc-29(kr208-RFP) (Richard et al, 2013), madd-4(kr270) (Pinan-Lucarré et al, 2014), madd-4(kr249) (Pinan-Lucarré et al, 2014), madd-4(ttTi103747) (Pinan-Lucarré et al, 2014), and madd-4(tr185) (Seetharaman et al, 2011);LGII, krSi2 [P unc-49 ::unc-49B-RFP] (Pinan-Lucarré et al, 2014);LGIII, unc-49(e407);LGIV, unc-5(e53), juIs1[P unc-25 ::snb-1-GFP], krSi9 ; and LGX, unc-6(ev400), nlg-1(ok259).…”

The ubiquitous and ancient ER membrane protein complex (EMC): tether or not?

“…The EMC has been implicated in several cellular processes. It has been implicated in ERAD (ER-associated degradation) ( Christianson et al , 2012; Jonikas et al , 2009; Richard et al , 2013) but the molecular mechanism for how EMC triggers ERAD has remained elusive. Emc6 contains a Rab5 interacting domain and has been shown to interact with Rab5A in humans during autophagosome formation ( Li et al , 2013).…”