Characterization of the C. elegans erlin homologue.

BACKGROUND: Erlins are highly conserved proteins associated with lipid rafts within the endoplasmic reticulum (ER). Biochemical studies in mammalian cell lines have shown that erlins are required for ER associated protein degradation (ERAD) of activated inositol-1,4,5-trisphosphate receptors (IP3Rs), implying that erlin proteins might negatively regulate IP3R signalling. In humans, loss of erlin function appears to cause progressive intellectual disability, motor dysfunction and joint contractures. However, it is unknown if defects in IP3R ERAD are the underlying cause of this disease phenotype, whether ERAD of activated IP3Rs is the only function of erlin proteins, and what role ERAD plays in regulating IP3R-dependent processes in the context of an intact animal or embryo. In this study, we characterize the erlin homologue of the nematode Caenorhabditis elegans and examine erlin function in vivo. We specifically set out to test whether C. elegans erlin modulates IP3R-dependent processes, such as egg laying, embryonic development and defecation rates. We also explore the possibility that erlin might play a more general role in the ERAD pathway of C. elegans. RESULTS: We first show that the C. elegans erlin homologue, ERL-1, is highly similar to mammalian erlins with respect to amino acid sequence, domain structure, biochemical properties and subcellular location. ERL-1 is present throughout the C. elegans embryo; in adult worms, ERL-1 appears restricted to the germline. The expression pattern of ERL-1 thus only partially overlaps with that of ITR-1, eliminating the possibility of ERL-1 being a ubiquitous and necessary regulator of ITR-1. We show that loss of ERL-1 does not affect overall phenotype, or alter brood size, embryonic development or defecation cycle length in either wild type or sensitized itr-1 mutant animals. Moreover we show that ERL-1 deficient worms respond normally to ER stress conditions, suggesting that ERL-1 is not an essential component of the general ERAD pathway. CONCLUSIONS: Although loss of erlin function apparently causes a strong phenotype in humans, no such effect is seen in C. elegans. C. elegans erlin does not appear to be a ubiquitous major modulator of IP3 receptor activity nor does erlin appear to play a major role in ERAD.

Distinct regions within the erlins are required for oligomerization and association with high molecular weight complexes.

The group of stomatin/prohibitin/flotillin/HflK/C (SPFH) domain-containing proteins comprise members of diverse subcellular localization and function. Association with detergent-resistant membranes (DRMs) and the propensity to form oligomers are two common properties of SPFH domain proteins and likely important for the function of these proteins. Our laboratory recently discovered two novel members of this protein group, which, based on their endoplasmic reticulum (ER) localization and association with DRMs, were named ER lipid raft-associated protein (erlin)-1 and -2. Here we characterized erlin oligomerization and identified domains within the erlins responsible for oligomerization and DRM association. Using co-immunoprecipitation and sucrose density gradient centrifugation approaches on endogenous and ectopically expressed erlin proteins, we found that they formed homo- and hetero-oligomers and were part of large multimeric complexes. These properties were independent of their DRM association. By analyzing truncation and point mutants of erlin-2 we discovered that interaction between erlin monomers (oligomerization) and association with high molecular weight complexes require distinct regions within the protein. Although oligomerization and DRM association were mediated by a region immediately downstream of the SPFH domain (residues 228-300), integration into high molecular weight complexes was absolutely dependent on a phenylalanine residue C-terminal of this region (Phe-305), which lies within a short stretch of hydrophobic residues. Our data demonstrate that lower order oligomerization and incorporation into multimeric complexes are two separate biochemical properties of the erlins, because they are mediated by distinct regions.

The SPFH domain-containing proteins: more than lipid raft markers.

Membrane microdomains with distinct lipid compositions, called lipid rafts, represent a potential mechanism for compartmentalizing cellular functions within the plane of biological membranes. SPFH domain-containing proteins are found in lipid raft microdomains in diverse cellular membranes. The functions of these proteins are just beginning to be elucidated. Recent advances in the understanding of structural features and their roles within lipid rafts include a potential function for SPFH proteins in the formation of membrane microdomains and lipid raft-associated processes, such as endocytosis and mechanosensation.

Reggie/flotillin proteins are organized into stable tetramers in membrane microdomains.

Reggie-1 and -2 proteins (flotillin-2 and -1 respectively) form their own type of non-caveolar membrane microdomains, which are involved in important cellular processes such as T-cell activation, phagocytosis and signalling mediated by the cellular prion protein and insulin; this is consistent with the notion that reggie microdomains promote protein assemblies and signalling. While it is generally known that membrane microdomains contain large multiprotein assemblies, the exact organization of reggie microdomains remains elusive. Using chemical cross-linking approaches, we have demonstrated that reggie complexes are composed of homo- and hetero-tetramers of reggie-1 and -2. Moreover, native reggie oligomers are indeed quite stable, since non-cross-linked tetramers are resistant to 8 M urea treatment. We also show that oligomerization requires the C-terminal but not the N-terminal halves of reggie-1 and -2. Using deletion constructs, we analysed the functional relevance of the three predicted coiled-coil stretches present in the C-terminus of reggie-1. We confirmed experimentally that reggie-1 tetramerization is dependent on the presence of coiled-coil 2 and, partially, of coiled-coil 1. Furthermore, since depletion of reggie-1 by siRNA (small interfering RNA) silencing induces proteasomal degradation of reggie-2, we conclude that the protein stability of reggie-2 depends on the presence of reggie-1. Our data indicate that the basic structural units of reggie microdomains are reggie homo- and hetero-tetramers, which are dependent on the presence of reggie-1.