The erlin2 T65I mutation inhibits erlin1/2 complex–mediated inositol 1,4,5-trisphosphate receptor ubiquitination and phosphatidylinositol 3-phosphate binding

Abstract

The erlin1/2 complex is a ~2MDa endoplasmic reticulum membrane-located ensemble of the ~40kDa type II membrane proteins erlin1 and erlin2. The best-defined function of this complex is to mediate the ubiquitination of activated inositol 1,4,5-trisphosphate receptors (IP3Rs) and their subsequent degradation. However, it remains unclear how mutations of the erlin1/2 complex affect its cellular function and cause cellular dysfunction and diseases such as hereditary spastic paraplegia. Here, we used gene editing to ablate erlin1 or erlin2 expression to better define their individual roles in the cell and examined the functional effects of a spastic paraplegia-linked mutation to erlin2 (threonine to isoleucine at position 65; T65I). Our results revealed that erlin2 is the dominant player in mediating the interaction between the erlin1/2 complex and IP3Rs, and that the T65I mutation dramatically inhibits this interaction and the ability of the erlin1/2 complex to promote IP3R ubiquitination and degradation. Remarkably, we also discovered that the erlin1/2 complex specifically binds to phosphatidylinositol 3-phosphate, that erlin2 binds this phospholipid much more strongly than does erlin1, that the binding is inhibited by T65I mutation of erlin2, and that multiple determinants within the erlin2 polypeptide comprise the phosphatidylinositol 3-phosphate-binding site.Overall, these results indicate that erlin2 is the primary mediator of the cellular roles of the erlin1/2 complex and that disease-linked mutations of erlin2 can affect both IP3R processing and lipid binding.

Dominant mutations in ITPR3 cause Charcot‐Marie‐Tooth disease

“…[10][11][12][13] Furthermore, the ER associated degradation pathway of activated IP 3 Rs is disrupted by inactivating variants in the genes ERLIN1, ERLIN2, and RNF170, causing hereditary spastic paraplegia and other neurodegenerative diseases. 14,15 IP 3 R3 itself has been implicated in apoptosis control, while alterations in its activity and/or expression levels drive oncogenesis and impact the survival of malignant cells. 16,17 In this study, we provide confirmatory evidence of the association of ITPR3 with CMT.…”

The emerging link between IP3 receptor turnover and Hereditary Spastic Paraplegia

“…RNF170 is recruited to activated IP 3 receptors by the erlin1/2 complex, to which it is constitutively bound (Fig. 1) [6,7]. In each of the four HSP cases [2], the mutation in RNF170 is likely inactivating, due to either a premature stop codon-mediated truncation, or a cysteine to arginine point mutation in the RING domain, C102R (Fig.…”

“…Additionally, several mutations to erlin1 and erlin2, the components of the erlin1/2 complex, have been linked to various neurological disorders, including uncomplicated and complicated HSPs, as well as lateral sclerosis [2,6]. For example, an erlin2 mutation (threonine to isoleucine at amino acid 65), which inhibits erlin1/2 complex recruitment to activated IP 3 receptors and thus IP 3 R1 processing, is linked to a HSP case [7]. Just like for RNF170, deletion of either erlin1 and erlin2 increases basal IP 3 R1 levels [7], indicating that any mutation that significantly impairs the function of the erlin1/2 complex-RNF170 module will affect normal IP 3 R1 processing and may thus cause neurodegeneration [2,6,8].…”

“…For example, an erlin2 mutation (threonine to isoleucine at amino acid 65), which inhibits erlin1/2 complex recruitment to activated IP 3 receptors and thus IP 3 R1 processing, is linked to a HSP case [7]. Just like for RNF170, deletion of either erlin1 and erlin2 increases basal IP 3 R1 levels [7], indicating that any mutation that significantly impairs the function of the erlin1/2 complex-RNF170 module will affect normal IP 3 R1 processing and may thus cause neurodegeneration [2,6,8]. Interestingly, several IP 3 R1 mutations have been identified that appear to be the cause of neurological disorders [2], but these tend to be spinocerebellar ataxias rather than HSPs [2], and whether any of these mutations affect IP 3 R receptor turnover remains to be determined.…”

“…IP 3 Rs form tetramers that act as ligand-gated Ca 2+ channels in the ER membrane [6]. The conformational change associated with channel opening triggers the binding of the erlin1/2 complex-RNF170 module (arrow) and, consequently, RNF170-mediated IP 3 R ubiquitination and proteasomal degradation [6,7]. The erlin1/2 complex is composed of ~40 erlin1 and erlin2 molecules and RNF170 is constitutively associated [6,7].…”