Amatoxins,Phallotoxins, Phallolysin,and Antamanide: The Biologically Active Components of Poisonous Amanita Mushroom

Multiple mechanisms for feedback control of cholesterol synthesis converge on the rate-limiting enzyme in the pathway, 3-hydroxy-3-methylglutaryl coenzyme A reductase. This complex feedback regulatory system is mediated by sterol and nonsterol metabolites of mevalonate, the immediate product of reductase activity. One mechanism for feedback control of reductase involves rapid degradation of the enzyme from membranes of the endoplasmic reticulum (ER). This degradation results from the accumulation of sterols in ER membranes, which triggers binding of reductase to ER membrane proteins called Insig-1 and Insig-2. Insig binding leads to the recruitment of a membrane-associated ubiquitin ligase called gp78 that initiates ubiquitination of reductase. Ubiquitinated reductase then becomes extracted from ER membranes and is delivered to cytosolic 26S proteasomes through an unknown mechanism that is mediated by the gp78-associated ATPase Valosin-containing protein/p97 and appears to be augmented by nonsterol isoprenoids. Here, we will highlight several advances that have led to the current view of mechanisms for sterol-accelerated, ER-associated degradation of reductase. In addition, we will discuss potential mechanisms for other aspects of the pathway such as selection of reductase for gp78-mediated ubiquitination, extraction of the ubiquitinated enzyme from ER membranes, and the contribution of Insig-mediated degradation to overall regulation of reductase in whole animals.     

Sterol-induced dislocation of 3-hydroxy-3-methylglutaryl coenzyme A reductase from membranes of permeabilized cells

The polytopic endoplasmic reticulum (ER)–localized enzyme 3-hydroxy-3-methylglutaryl CoA reductase catalyzes a rate-limiting step in the synthesis of cholesterol and nonsterol isoprenoids. Excess sterols cause the reductase to bind to ER membrane proteins called Insig-1 and Insig-2, which are carriers for the ubiquitin ligases gp78 and Trc8. The resulting gp78/Trc8-mediated ubiquitination of reductase marks it for recognition by VCP/p97, an ATPase that mediates subsequent dislocation of reductase from ER membranes into the cytosol for proteasomal degradation. Here we report that in vitro additions of the oxysterol 25-hydroxycholesterol (25-HC), exogenous cytosol, and ATP trigger dislocation of ubiquitinated and full-length forms of reductase from membranes of permeabilized cells. In addition, the sterol-regulated reaction requires the action of Insigs, is stimulated by reagents that replace 25-HC in accelerating reductase degradation in intact cells, and is augmented by the nonsterol isoprenoid geranylgeraniol. Finally, pharmacologic inhibition of deubiquitinating enzymes markedly enhances sterol-dependent ubiquitination of reductase in membranes of permeabilized cells, leading to enhanced dislocation of the enzyme. Considered together, these results establish permeabilized cells as a viable system in which to elucidate mechanisms for postubiquitination steps in sterol-accelerated degradation of reductase.     

Feedback regulation of cholesterol synthesis: sterol-accelerated ubiquitination and degradation of HMG CoA reductase

3-hydroxy-3-methylglutaryl coenzyme A (HMG CoA) reductase produces mevalonate, an important intermediate in the synthesis of cholesterol and essential nonsterol isoprenoids. The reductase is subject to an exorbitant amount of feedback control through multiple mechanisms that are mediated by sterol and nonsterol end-products of mevalonate metabolism. Here, I will discuss recent advances that shed light on one mechanism for control of reductase, which involves rapid degradation of the enzyme. Accumulation of certain sterols triggers binding of reductase to endoplasmic reticulum (ER) membrane proteins called Insig-1 and Insig-2. Reductase-Insig binding results in recruitment of a membrane-associated ubiquitin ligase called gp78, which initiates ubiquitination of reductase. This ubiquitination is an obligatory reaction for recognition and degradation of reductase from ER membranes by cytosolic 26S proteasomes. Thus, sterol-accelerated degradation of reductase represents an example of how a general cellular process (ER-associated degradation) is used to control an important metabolic pathway (cholesterol synthesis).     

Lipid-regulated degradation of HMG-CoA reductase and Insig-1 through distinct mechanisms in insect cells

In mammalian cells, levels of the integral membrane proteins 3-hydroxy-3-methylglutaryl-CoA reductase and Insig-1 are controlled by lipid-regulated endoplasmic reticulum-associated degradation (ERAD). The ERAD of reductase slows a rate-limiting step in cholesterol synthesis and results from sterol-induced binding of its membrane domain to Insig-1 and the highly related Insig-2 protein. Insig binding bridges reductase to ubiquitin ligases that facilitate its ubiquitination, thereby marking the protein for cytosolic dislocation and proteasomal degradation. In contrast to reductase, Insig-1 is subjected to ERAD in lipid-deprived cells. Sterols block this ERAD by inhibiting Insig-1 ubiquitination, whereas unsaturated fatty acids block the reaction by preventing the protein''s cytosolic dislocation. In previous studies, we found that the membrane domain of mammalian reductase was subjected to ERAD in Drosophila S2 cells. This ERAD was appropriately accelerated by sterols and required the action of Insigs, which bridged reductase to a Drosophila ubiquitin ligase. We now report reconstitution of mammalian Insig-1 ERAD in S2 cells. The ERAD of Insig-1 in S2 cells mimics the reaction that occurs in mammalian cells with regard to its inhibition by either sterols or unsaturated fatty acids. Genetic and pharmacologic manipulations coupled with subcellular fractionation indicate that Insig-1 and reductase are degraded through distinct mechanisms that are mediated by different ubiquitin ligase complexes. Together, these results establish Drosophila S2 cells as a model system to elucidate mechanisms through which lipid constituents of cell membranes (i.e., sterols and fatty acids) modulate the ERAD of Insig-1 and reductase.     

Dislocation of HMG-CoA Reductase and Insig-1, Two Polytopic Endoplasmic Reticulum Proteins,En Route to Proteasomal Degradation

The endoplasmic reticulum (ER) glycoprotein HMG-CoA reductase (HMGR) catalyzes the rate-limiting step in sterols biosynthesis. Mammalian HMGR is ubiquitinated and degraded by the proteasome when sterols accumulate in cells, representing the best example for metabolically controlled ER-associated degradation (ERAD). This regulated degradation involves the short-lived ER protein Insig-1. Here, we investigated the dislocation of these ERAD substrates to the cytosol en route to proteasomal degradation. We show that the tagged HMGR membrane region, HMG₃₅₀-HA, the endogenous HMGR, and Insig-1-Myc, all polytopic membrane proteins, dislocate to the cytosol as intact full-length polypeptides. Dislocation of HMG₃₅₀-HA and Insig-1-Myc requires metabolic energy and involves the AAA-ATPase p97/VCP. Sterols stimulate HMG₃₅₀-HA and HMGR release to the cytosol concurrent with removal of their N-glycan by cytosolic peptide:N-glycanase. Sterols neither accelerate dislocation nor stimulate deglycosylation of ubiquitination-defective HMG₃₅₀-HA^{(K89 + 248R)} mutant. Dislocation of HMG₃₅₀-HA depends on Insig-1-Myc, whose dislocation and degradation are sterol independent. Coimmunoprecipitation experiments demonstrate sterol-stimulated association between HMG₃₅₀-HA and Insig-1-Myc. Sterols do not enhance binding to Insig-1-Myc of HMG₃₅₀-HA mutated in its sterol-sensing domain or of HMG₃₅₀-HA^{(K89 + 248R)}. Wild-type HMG₃₅₀-HA and Insig-1-Myc coimmunoprecipitate from the soluble fraction only when both proteins were coexpressed in the same cell, indicating their encounter before or during dislocation, raising the possibility that they are dislocated as a tightly bound complex.     

Ubiquitination of 3-hydroxy-3-methylglutaryl-CoA reductase in permeabilized cells mediated by cytosolic E1 and a putative membrane-bound ubiquitin ligase

The endoplasmic reticulum (ER) enzyme, 3-hydroxy-3-methylglutaryl-CoA reductase, catalyzes the production of mevalonate, a rate-controlling step in cholesterol biosynthesis. Excess sterols promote ubiquitination and subsequent degradation of reductase as part of a negative feedback regulatory mechanism. To characterize the process in more detail, we here report the development of a permeabilized cell system that supports reductase ubiquitination stimulated by the addition of sterols in vitro. Sterol-dependent ubiquitination of reductase in permeabilized cells is dependent upon exogenous cytosol, ATP, and either Insig-1 or Insig-2, two membrane-bound ER proteins shown previously to mediate sterol regulation of reductase degradation in intact cells. Oxysterols, but not cholesterol, promote reductase ubiquitination under our conditions. Finally, we show that ubiquitin-activating enzyme (E1) can efficiently replace cytosol to ubiquitinate reductase in response to sterol treatment, suggesting that other molecules required for ubiquitination of reductase, such as the ubiquitin-conjugating and -ligating enzymes (E2 and E3), are localized to ER membranes.     

Sequential Actions of the AAA-ATPase Valosin-containing Protein (VCP)/p97 and the Proteasome 19 S Regulatory Particle in Sterol-accelerated,Endoplasmic Reticulum (ER)-associated Degradation of 3-Hydroxy-3-methylglutaryl-coenzyme A Reductase

Accelerated endoplasmic reticulum (ER)-associated degradation (ERAD) of the cholesterol biosynthetic enzyme 3-hydroxy-3-methylglutaryl-coenzyme A reductase results from its sterol-induced binding to ER membrane proteins called Insig-1 and Insig-2. This binding allows for subsequent ubiquitination of reductase by Insig-associated ubiquitin ligases. Once ubiquitinated, reductase becomes dislocated from ER membranes into the cytosol for degradation by 26 S proteasomes through poorly defined reactions mediated by the AAA-ATPase valosin-containing protein (VCP)/p97 and augmented by the nonsterol isoprenoid geranylgeraniol. Here, we report that the oxysterol 25-hydroxycholesterol and geranylgeraniol combine to trigger extraction of reductase across ER membranes prior to its cytosolic release. This conclusion was drawn from studies utilizing a novel assay that measures membrane extraction of reductase by determining susceptibility of a lumenal epitope in the enzyme to in vitro protease digestion. Susceptibility of the lumenal epitope to protease digestion and thus membrane extraction of reductase were tightly regulated by 25-hydroxycholesterol and geranylgeraniol. The reaction was inhibited by RNA interference-mediated knockdown of either Insigs or VCP/p97. In contrast, reductase continued to become membrane-extracted, but not cytosolically dislocated, in cells deficient for AAA-ATPases of the proteasome 19 S regulatory particle. These findings establish sequential roles for VCP/p97 and the 19 S regulatory particle in the sterol-accelerated ERAD of reductase that may be applicable to the ERAD of other substrates.     

Accelerated degradation of HMG CoA reductase mediated by binding of insig-1 to its sterol-sensing domain

Sterols accelerate degradation of the ER enzyme 3-hydroxy-3-methylglutaryl CoA reductase (HMG CoA reductase), which catalyzes a rate-controlling step in cholesterol biosynthesis. This degradation contributes to feedback inhibition of synthesis of cholesterol and nonsterol isoprenoids. Here, we show that degradation of HMG CoA reductase is accelerated by the sterol-induced binding of its sterol-sensing domain to the ER protein insig-1. Accelerated degradation is inhibited by overexpression of the sterol-sensing domain of SREBP cleavage-activating protein (SCAP), suggesting that both proteins bind to the same site on insig-1. Whereas insig-1 binding to SCAP leads to ER retention, insig-1 binding to HMG CoA reductase leads to accelerated degradation that is blocked by proteasome inhibitors. Insig-1 appears to play an essential role in the sterol-mediated trafficking of two proteins with sterol-sensing domains, HMG CoA reductase and SCAP.     

Insig required for sterol-mediated inhibition of Scap/SREBP binding to COPII proteins in vitro

When added to living cells, sterols such as cholesterol and 25-hydroxycholesterol block the lateral movement of sterol regulatory element-binding proteins (SREBPs) into COPII-coated vesicles on endoplasmic reticulum (ER) membranes and thereby prevent the SREBPs from reaching the Golgi complex for processing to the mature forms that activate cholesterol synthesis. Sorting of SREBPs into COPII vesicles is mediated by Sar1 and the coat proteins Sec23 and Sec24. Here, we explore the mechanism of sterol inhibition in vitro through use of protein pull-down assays. We show that addition of cholesterol or 25-hydroxycholesterol to microsomal membranes in vitro blocks Sar1-dependent binding of the Sec23/24 complex to Scap, the SREBP escort protein. This in vitro inhibition is dependent on the presence of Insig-1, an ER resident protein that is necessary for sterol-mediated inhibition of Scap/SREBP transport in intact cells. Sec23/24 binding to Scap requires the hexapeptide sequence MELADL located in a cytoplasmic loop of Scap. This hexapeptide acts as a sterol-regulated ER sorting signal. These studies define the biochemical parameters responsible for regulated sorting of an ER membrane protein into COPII-coated vesicles.     

Insig-dependent ubiquitination and degradation of mammalian 3-hydroxy-3-methylglutaryl-CoA reductase stimulated by sterols and geranylgeraniol

The endoplasmic reticulum enzyme 3-hydroxy-3-methylglutaryl-CoA reductase produces mevalonate, which is converted to sterols and to other products, including geranylgeraniol groups attached to proteins. The enzyme is known to be ubiquitinated and rapidly degraded when sterols and nonsterol end products of mevalonate metabolism accumulate in cells. Here, we use RNA interference to show that sterol-accelerated ubiquitination of reductase requires Insig-1 and Insig-2, membrane-bound proteins of the endoplasmic reticulum that were shown previously to accelerate degradation of reductase when overexpressed by transfection. Alanine substitution experiments reveal that binding of reductase to Insigs and subsequent ubiquitination require the tetrapeptide sequence YIYF in the second membrane-spanning helix of reductase. The YIYF peptide is also found in the sterol-sensing domain of SCAP, another protein that binds to Insigs in a sterol-stimulated fashion. When lysine 248 of reductase is substituted with arginine, Insig binding persists, but the reductase is no longer ubiquitinated and degradation is markedly slowed. Lysine 248 is predicted to lie immediately adjacent to a membrane-spanning helix, suggesting that a membrane-bound ubiquitin transferase is responsible. Finally, we show that Insig-dependent, sterol-stimulated degradation of reductase is further accelerated when cells are also supplied with the 20-carbon isoprenoid geranylgeraniol, but not the 15-carbon farnesol, raising the possibility that the nonsterol potentiator of reductase regulation is a geranylgeranylated protein.     

Membrane-bound domain of HMG CoA reductase is required for sterol-enhanced degradation of the enzyme

3-Hydroxy-3-methylglutaryl coenzyme A reductase (HMG CoA reductase) is a single polypeptide chain with two contiguous domains: a soluble domain (548 amino acids) that catalyzes the rate-controlling step in cholesterol synthesis and a membrane-bound domain (339 amino acids) that anchors the protein to the endoplasmic reticulum (ER). HMG CoA reductase is degraded at least 10-fold more rapidly than other ER proteins; degradation is accelerated in the presence of cholesterol. To understand this controlled degradation, we transfected reductase-deficient Chinese hamster ovary (CHO) cells with a plasmid expression vector containing a reductase cDNA that lacks the segment encoding the membrane domain. The plasmid produced a truncated reductase (37 kd smaller than normal) that was enzymatically active with normal kinetics; most of the truncated enzyme was found in the cytosol. The truncated enzyme was degraded one-fifth as fast as the holoenzyme; degradation was no longer accelerated by sterols. We conclude that the membrane-bound domain of reductase plays a crucial role in the rapid and regulated degradation of this ER protein.