ReviewWhen worlds collide: IP3 receptors and the ERAD pathway
Section snippets
IP3 receptors and their activation
IP3 receptors are large (∼2700 amino acid) endoplasmic reticulum (ER) membrane proteins which form tetrameric channels that govern the release of Ca2+ stored within the ER lumen of vertebrate cells (Fig. 1) [1], [2], [3]. They are named for their ability to bind to and be opened by the second messenger IP3, which is generated at the plasma membrane in response to cell surface receptor activation. Thus, IP3 receptors are pivotal in signaling pathways that couple extracellular hormones,
IP3 receptor down-regulation
In 1991 it was discovered in mammalian cell lines that in response to activation of certain IP3-generating cell surface receptors, IP3 receptors are “down-regulated”, i.e. there is a rapid and dramatic decline in cellular IP3 receptor content [9]. Typically, this decline is >50%, with half-maximal effect at 30–60 min [10], [11], [12], [13], [14], but is particularly marked in αT3-1 anterior pituitary cells, in which gonadotropin-releasing hormone (GnRH) receptor activation down-regulates IP3R1
The UPP
The UPP is currently the focus of intense interest, since it is now known to be the major route of protein degradation in eukaryotic cells and mediates the selective destruction of many important proteins, including signaling pathway proteins and regulators of the cell cycle and transcription [26]. In addition, it is responsible for “quality control” in the ER; i.e., the selective degradation of misfolded proteins, and of unused subunits of multimeric protein complexes, in a process known as
ERAD
In addition to being a Ca2+ store, the ER is of course, also the synthesis site of membrane and secreted proteins, which account for ∼1/3 of all proteins [27]. It has emerged in recent years that a sophisticated system, ERAD, exists in eukaryotes for the disposal of proteins that do not fold properly or which cannot find their normal binding partners (Fig. 2) [27]. Intriguingly, some ER-resident proteins that are stable under normal conditions are also processed in this manner, the prototype
Are IP3 receptors ERAD substrates?
Evidence that IP3 receptors are UPP substrates came from experiments showing that IP3 receptors are polyubiquitinated, and that proteasome inhibitors block their down-regulation [12], [16], [18], [25], [40]. Obviously, their location in the ER immediately suggested that IP3 receptors could be targeted by the ERAD pathway and subsequent studies supported this view – an E2 that ubiquitinates IP3 receptors is ubc7 [40], an enzyme implicated in both yeast and mammalian ERAD pathways [31], [36], and
IP3 receptor ubiquitination is surprisingly complex
A fundamental unanswered question for most UPP substrates concerns where they are ubiquitinated and with what, and only recently, with the advent of mass spectrometry-based technologies [33], [43], [44], [45], has it been possible to address this question. Application of this approach to IP3R1 isolated from GnRH-stimulated αT3-1 cells, showed that at least 11 of IP3R1's 167 lysines can be sites of ubiquitination (Fig. 3A), that of the attached ubiquitin moieties, at least ∼40% are
Special delivery
To contemplate how ubiquitinated IP3 receptors might be degraded by the proteasome is quite daunting. To enter the catalytic core of the proteasome, proteins must first be unfolded [27], [28], yet IP3 receptor subunits have 6 TM domains and in their native state are tightly associated into tetramers ∼1 MDa in size (Fig. 1A). Two new pieces of data appear to speak to this issue. First, in contrast to many model ERAD substrates [49], [50], IP3 receptors are not released into the cytosol prior to
The SPFH1/2 complex and selection of activated IP3 receptors for ERAD
Several proteins, including p97, associate with IP3 receptors in an activation-dependent manner [41] and most recently it has been demonstrated that SPFH1 and SPFH2 (erlin 1 and erlin 2; see Section 9) [51], also have this property [42], [52]. Fig. 4A–C shows the essential features of these two proteins – that they associate rapidly with IP3 receptors in a manner that precedes maximal IP3 receptor ubiquitination and association of p97, that they are type II ER membrane glycoproteins, and that
SPFH domain-containing proteins
SPFH1 and SPFH2 belong to a family of ∼100 mammalian proteins that contain an “SPFH” domain, an ∼250 amino acid motif named because of minor sequence similarities in the proteins Stomatin, Prohibitin, Flotillin (reggie), and HflC/K [51]. SPFH domain-containing proteins share some similarities, including localization to cholesterol-rich, detergent-resistant membranes (DRMs), and assembly into large (>1 MDa) oligomeric structures [51]. To date, however, no universal function has been attributed to
Conclusions and perspectives
That a fraction of activated IP3 receptors are hived off for ERAD is both surprising and intriguing. The cell is inactivating IP3 gated channels in a very radical manner – by degradation as opposed to a reversible modification. This could represent a finely tuned mechanism to suppress Ca2+ signaling. Alternatively, it could be more by accident than design – it is possible that during the activation process, IP3 receptors “accidentally” expose regions (e.g. hydrophobic patches) that makes them
Acknowledgements
The authors wish to apologize to those whose work was omitted due to space constraints, and wish to thank the National Institutes of Health, the Pharmaceutical Research and Manufacturers of America Foundation, and the American Heart Association for financial support.
References (64)
- et al.
IP3 receptors: the search for structure
Trends Biochem. Sci.
(2004) - et al.
Ligand-induced conformational changes via flexible linkers in the amino-terminal region of the inositol 1,4,5-trisphosphate receptor
J. Mol. Biol.
(2007) - et al.
Three-dimensional rearrangements within inositol 1,4,5-trisphosphate receptor by calcium
J. Biol. Chem.
(2003) - et al.
The role of the S4-S5 linker and C-terminal tail in inositol 1,4,5-trisphosphate receptor function
J. Biol. Chem.
(2006) - et al.
Molecular characterization of the inositol 1,4,5-trisphosphate receptor pore-forming segment
J. Biol. Chem.
(2008) - et al.
Inositol 1,4,5-trisphosphate receptor contains multiple cavities and L-shaped ligand-binding domains
J. Mol. Biol.
(2004) - et al.
Chronic muscarinic stimulation of SH-SY5Y neuroblastoma cells suppresses inositol 1,4,5-trisphosphate action: Parallel inhibition of inositol 1,4,5-trisphosphate-induced Ca2+ mobilization and inositol 1,4,5-trisphosphate binding
J. Biol. Chem.
(1991) - et al.
Muscarinic receptor activation down-regulates the type I inositol 1,4,5-trisphosphate receptor by accelerating its degradation
J. Biol. Chem.
(1994) Type I, II and III inositol 1,4,5-trisphosphate receptors are unequally susceptible to down-regulation and are expressed in markedly different proportions in different cell types
J. Biol. Chem.
(1995)- et al.
Angiotensin II-induced down-regulation of inositol trisphosphate receptors in WB rat liver epithelial cells: Evidence for the involvement of the proteasome pathway
J. Biol. Chem.
(1997)
Regulated ubiquitination of proteins in GPCR-initiated signalling pathways
Trends Pharmacol. Sci.
Rapid down-regulation of the type I inositol 1,4,5-trisphosphate receptor and desensitization of gonadotropin-releasing hormone-mediated Ca2+ responses in αT3-1 gonadotropes
J. Biol. Chem.
Ubiquitination and proteasomal degradation of endogenous and exogenous inositol 1,4,5 trisphosphate receptors in αT3-1 anterior pituitary cells
J. Biol. Chem.
Secretagogues cause the ubiquitination and down-regulation of inositol 1,4,5 trisphosphate receptors in rat pancreatic acinar cells
Gastroenterology
Expression of inositol 1,4,5 trisphosphate receptors in mouse oocytes and early embryos: the type I isoform is upregulated in oocytes and down-regulated after fertilization
Dev. Biol.
Down-regulation of the inositol 1,4,5-trisphosphate receptor in mouse eggs following fertilization or parthenogenetic activation
Dev. Biol.
Is an elevated concentration of acinar cytosolic free ionized calcium the trigger for acute pancreatitis?
Lancet
The role of inositol 1,4,5-trisphosphate receptors in the regulation of bile secretion in health and disease
Biochem. Biophys. Res. Commun.
Ubiquitin ligases, critical mediators of endoplasmic reticulum-associated degradation
Semin. Cell Dev. Biol
Differential regulation of EGF receptor internalization and degradation by multiubiquitination within the kinase domain
Mol. Cell
Distinct ubiquitin-ligase complexes define convergent pathways for the degradation of ER proteins
Cell
Sequential quality-control checkpoints triage misfolded cystic fibrosis transmembrane conductance regulator
Cell
Ubiquitin ligase Kf-1 is involved in the endoplasmic reticulum-associated degradation pathway
Biochem. Biophys. Res. Commun.
Inositol 1,4,5 trisphosphate receptor ubiquitination is mediated by mammalian Ubc7, a component of the Endoplasmic Reticulum-Associated Degradation pathway, and is inhibited by chelation of intracellular Zn2+
J. Biol. Chem.
Involvement of the p97-Ufd1-Npl4 complex in the regulated endoplasmic reticulum-associated degradation of inositol 1,4,5-trisphosphate receptors
J. Biol. Chem.
An endoplasmic reticulum (ER) membrane complex composed of SPFH1 and SPFH2 mediates the ER-associated degradation of inositol 1,4,5-trisphosphate receptors
J. Biol. Chem.
Mass spectral analysis of type I inositol 1,4,5-trisphosphate receptor ubiquitination
J. Biol. Chem.
Certain pairs of ubiquitin-conjugating enzymes (E2s) and ubiquitin-protein ligases (E3s) synthesize nondegradable forked ubiquitin chains containing all possible isopeptide linkages
J. Biol. Chem.
Ubiquitin chain editing revealed by polyubiquitin linkage-specific antibodies
Cell
Dissecting the ER-associated degradation of a misfolded polytopic membrane protein
Cell
The SPFH domain-containing proteins: more than lipid raft markers
Trends Cell Biol.
SPFH2 mediates the ERAD of IP3 receptors and other substrates in mammalian cells
J. Biol. Chem.
Cited by (49)
Non-inositol 1,4,5-trisphosphate (IP<inf>3</inf>) receptor IP<inf>3</inf>-binding proteins
2023, Biochimica et Biophysica Acta - Molecular Cell ResearchComprehensive analysis of ceRNA networks in HPV16- and HPV18-mediated cervical cancers reveals XIST as a pivotal competing endogenous RNA
2021, Biochimica et Biophysica Acta - Molecular Basis of DiseaseCitation Excerpt :For instance, as an important regulator of distinct biological processes like proliferation, invasion, migration ability, and apoptosis, ERLIN2 stood out as a key gene in HPV18-mediated CESC. The endoplasmic reticulum (ER) lipid raft associated 2 (ERLIN2) (also known as SPFH2 or C8ORF2) is an ER-microtubule-binding protein, presenting a pivotal role in inositol 1,4,5-triphosphate (IP3) signaling by mediating ER-associated degradation of activated IP3 receptors (ITPRs) [78]. ITPRs are intracellular calcium release channels located in the ER of virtually every cell.
Pathophysiological consequences of isoform-specific IP <inf>3</inf> receptor mutations
2018, Biochimica et Biophysica Acta - Molecular Cell Research“Mallostery”—ligand-dependent protein misfolding enables physiological regulation by ERAD
2018, Journal of Biological ChemistryThe stability and expression level of Bok are governed by binding to inositol 1,4,5-trisphosphate receptors
2016, Journal of Biological ChemistryChapter 4 - Inositol 1,4,5-Trisphosphate Receptor Ubiquitination
2016, Progress in Molecular Biology and Translational Science
- 1
Present address: Department of Biology, Stanford University, Palo Alto, CA 94305, USA.