About 45 years ago, Jonathan Hodgkin isolated many extragenic suppressors of the
unc-17(
e245) mutation. These suppressors defined 3 genes:
sup-1,
sup-2, and
sup-8. Suppressors at each of these loci had no obvious phenotype on their own, but caused strong dominant suppression of
unc-17(
e245) phenotypes. All 3 suppressors were specific for the allele
e245, which later proved to be a missense G347R change in TM9 of the UNC-17 acetylcholine transporter (Alfonso et al., 1993). Subsequently,
sup-8 was cloned and found to be a dominant allele of
snb-1, encoding the synaptic protein synaptobrevin (Sandoval et al., 2006). When
sup-1 was cloned it was found to encode a small single-pass transmembrane protein (Mathews et al., 2012). Suppressor mutations at both loci led to a change of a neutral amino acid to a (negatively charged) acidic amino acid in the middle of each protein's transmembrane domain (I97D for
sup-8, and G84E for
sup-1). Suppression can therefore be explained by compensatory charge interactions between transmembrane helices. We now report analysis of
sup-2, which was cloned by a combination of 3-factor mapping and brute-force sequencing.
sup-2(
e997) is a semi-dominant loss-of-function allele of F09B9.3, now named
erd-2.1. This gene encodes a 7-pass transmembrane protein homologous to the universal eukaryotic ER protein retention receptor, which acts to retrieve endoplasmic reticulum proteins from the Golgi complex and shuttles between these compartments. The
sup-2(
e997) mutation converts a (neutral) valine to a (negatively charged) glutamic acid (V186E) in TM7 of ERD-2.1. F09B9.3 has a paralog in C. elegans and other nematodes: C28H8.4, now named
erd-2.2. Transgenes carrying either
erd-2.1(V186E) or
erd-2.2(V186E) suppressed
unc-17(
e245). Redundancy between these genes was earlier demonstrated by Tischler et al. (2006): RNAi knockdown of either single gene had no effect but double knockdown was synthetic lethal. We confirmed these results, and found that
erd-2.2(RNAi) alone had a lethal effect on
sup-2(
e997). Thus,
e997 results in loss of ERD-2.1 function. This loss of function may result in its mislocalization and thereby permit interaction with synaptic UNC-17(G347R).