Rhomboid proteases: familiar features in unfamiliar phases.

In this issue of Molecular Cell, Strisovsky et al. (2009) identify a sequence motif underlying cleavage site specificity for the rhomboid proteases. This sheds light on potential mechanisms by which intramembrane-cleaving proteases cleave their substrates.

The site-2 protease.

The site-2 protease (S2P) is an unusually-hydrophobic integral membrane protease. It cleaves its substrates, which are membrane-bound transcription factors, within membrane-spanning helices. Although structural information for S2P from animals is lacking, the available data suggest that cleavage may occur at or within the lipid bilayer. In mammalian cells, S2P is essential owing to its activation of the sterol regulatory element binding proteins (SREBPs); in the absence of exogenous lipid, cells lacking S2P cannot survive. S2P is also important in the endoplasmic reticulum (ER) stress response, activating several different membrane-bound transcription factors. Human patients harboring reduction-of-function mutations in S2P exhibit an array of pathologies ranging from skin defects to neurological abnormalities. Surprisingly, Drosophila melanogaster lacking S2P are viable and fertile. This article is part of a Special Issue entitled: Intramembrane Proteases.

An ARC light on lipid metabolism.

The SREBP pathway plays a central role in the regulation of lipid metabolism. In a recent letter, Yang et al. present a comprehensive series of experiments, spanning a wide range of disciplines, that identify ARC105 as a component of the ARC complex that interacts directly with SREBP and is necessary for SREBP function (Yang et al., 2006).

Fatty acid auxotrophy in Drosophila larvae lacking SREBP.

SREBPs are membrane bound transcription factors that are crucial for normal lipid synthesis in animal cells. Here, we show that Drosophila lacking dSREBP die before the third larval instar. Mutant larvae exhibit pronounced growth defects prior to lethality, along with substantial deficits in the transcription of genes required for fatty acid synthesis. Compared to wild-type larvae, mutants contain markedly less fatty acid, although its composition is unaltered. Dietary supplementation with fatty acids rescues mutants to adulthood. The most effective fatty acid, oleate, rescues 80% of homozygotes. Rescue by dSREBP requires expression only in fat body and gut. Larvae expressing dSREBP prior to pupariation complete development and are viable as adults even when dSREBP expression is subsequently extinguished. The role, if any, of dSREBP in adults is not yet apparent. These data indicate that dSREBP deficiency renders Drosophila larvae auxotrophic for fatty acids.

Alternative processing of sterol regulatory element binding protein during larval development in Drosophila melanogaster.

Sterol regulatory element binding protein (SREBP) is a major transcriptional regulator of lipid metabolism. Nuclear Drosophila SREBP (dSREBP) is essential for larval development in Drosophila melanogaster but dispensable in adults. dSREBP(-) larvae die at second instar owing to loss of dSREBP-mediated transcription but survive to adulthood when fed fatty acids. Activation of SREBP requires two separate cleavages. Site-1 protease (S1P) cleaves in the luminal loop of the membrane-bound SREBP precursor, cutting it in two. The NH(2)- and COOH-terminal domains remain membrane bound owing to their single membrane-spanning helices. The NH(2)-terminal cleavage product is the substrate for site-2 protease (S2P), which cleaves within its membrane-spanning helix to release the transcription factor. In mice, loss of S1P is lethal but the consequences of loss of S2P in animals remain undefined. All known functions of SREBP require its cleavage by S2P. We isolated Drosophila mutants that eliminate all dS2P function (dS2P(-)). Unexpectedly, larvae lacking dS2P are viable. They are deficient in transcription of some dSREBP target genes but less so than larvae lacking dSREBP. Despite loss of dS2P, dSREBP is processed in mutant larvae. Therefore, larvae have an alternative cleavage mechanism for producing transcriptionally active dSREBP, and this permits survival of dS2P mutants.

Desmosterol can replace cholesterol in sustaining cell proliferation and regulating the SREBP pathway in a sterol-Delta24-reductase-deficient cell line.

Cholesterol homoeostasis is critical for cell viability and proliferation. The SREBP (sterol regulatory element-binding protein) pathway is crucial for the maintenance of cholesterol homoeostasis. This pathway is controlled by cholesterol and cholesterol-derived oxysterols. J774 cells cannot convert desmosterol into cholesterol, a defect resulting from the absence of mRNA for sterol-Delta24-reductase. Using J774 cells, we addressed the capacity of desmosterol to replace cholesterol in sustaining cell proliferation and regulating the SREBP pathway. J774 cells were able to grow indefinitely after the virtually total replacement of cholesterol by desmosterol (J774-D cells). Inhibition of sterol biosynthesis with lovastatin suppressed J774-D cell proliferation. Desmosterol prevented this effect, but its analogue, cholest-5,22-trans-dien-3beta-ol, did not. Addition of desmosterol inhibited processing of SREBP-1 and -2 and also reduced the expression of SREBP-targeted genes. As occurs in cholesterol-containing cells, 25-hydroxycholesterol was more potent than desmosterol or cholesterol in suppressing these processes. Moreover, desmosterol addition enhanced the expression of Abca1 and Srebf1c, two LXR (liver X receptor)-targeted genes. To test the ability of endogenously produced desmosterol to regulate gene expression, J774-D cells were pretreated with lovastatin to inhibit sterol biosynthesis. After removal of the inhibitor the expression of SREBP-targeted genes decreased and that of an LXR-targeted gene increased, reaching control levels. Our results demonstrate that the virtually complete replacement of cholesterol by desmosterol is compatible with cell growth and the functioning of the SREBP pathway. In these cells, desmosterol suppresses SREBP processing and targeted gene expression, and it is especially effective activating LXR-targeted genes.

The SREBP pathway in Drosophila: regulation by palmitate, not sterols.

In mammals, synthesis of cholesterol and unsaturated fatty acids is controlled by SREBPs, a family of membrane-bound transcription factors. Here, we show that the Drosophila genome encodes all components of the SREBP pathway, including a single SREBP (dSREBP), SREBP cleavage-activating protein (dSCAP), and the two proteases that process SREBP at sites 1 and 2 to release the nuclear fragment. In cultured Drosophila S2 cells, dSREBP is processed at sites 1 and 2, and the liberated fragment increases mRNAs encoding enzymes of fatty acid biosynthesis, but not sterol or isoprenoid biosynthesis. Processing requires dSCAP, but is not inhibited by sterols as in mammals. Instead, dSREBP processing is blocked by palmitic acid. These findings suggest that the ancestral SREBP pathway functions to maintain membrane integrity rather than to control cholesterol homeostasis.

COPI activity coupled with fatty acid biosynthesis is required for viral replication.

During infection by diverse viral families, RNA replication occurs on the surface of virally induced cytoplasmic membranes of cellular origin. How this process is regulated, and which cellular factors are required, has been unclear. Moreover, the host-pathogen interactions that facilitate the formation of this new compartment might represent critical determinants of viral pathogenesis, and their elucidation may lead to novel insights into the coordination of vesicular trafficking events during infection. Here we show that in Drosophila cells, Drosophila C virus remodels the Golgi apparatus and forms a novel vesicular compartment, on the surface of which viral RNA replication takes place. Using genome-wide RNA interference screening, we found that this step in the viral lifecycle requires at least two host encoded pathways: the coat protein complex I (COPI) coatamer and fatty acid biosynthesis. Our results integrate, clarify, and extend numerous observations concerning the cell biology of viral replication, allowing us to conclude that the coupling of new cellular membrane formation with the budding of these vesicles from the Golgi apparatus allows for the regulated generation of this new virogenic organelle, which is essential for viral replication. Additionally, because these pathways are also limiting in flies and in human cells infected with the related RNA virus poliovirus, they may represent novel targets for antiviral therapies.

Regulated intramembrane proteolysis: from the endoplasmic reticulum to the nucleus.

Regulated intramembrane proteolysis (Rip) is an ancient and widespread process by which cells transmit information from one compartment (the endoplasmic reticulum) to another (the nucleus). Two separate cleavages that are carried out by two separate proteases are required for Rip. The first protease cleaves its protein substrate within an extracytoplasmic domain; the second cleaves it within a membrane-spanning domain, releasing a functionally active fragment of the target protein. In eukaryotes, examples of Rip can be divided into two classes, according to the proteases that are involved and the orientation of the substrates with the membrane. Class 1 Rip involves type 1 transmembrane proteins and requires presenilin for cleavage within a membrane-spanning domain. In Class 2 Rip, the highly hydrophobic metalloprotease, site-2 protease, is required for cleavage within a membrane-spanning domain and substrates are type 2 transmembrane proteins. Both classes of Rip are implicated in diseases that are important in modern societies, such as hyperlipidaemias (via the sterol regulatory element binding protein pathway) and Alzheimer's disease (via processing of the amyloid precursor protein.)

Control of lipid metabolism by regulated intramembrane proteolysis of sterol regulatory element binding proteins (SREBPs).

In mammalian cells, the supply of lipids is co-ordinated with demand through the transcriptional control of genes encoding proteins required for synthesis or uptake. The sterol regulatory element binding proteins (SREBPs) are responsible for increased transcription of these genes when lipid level fall. Mammals have three SREBPs (-1a, -1c and -2), which are the products of two distinct genes. Synthesized as approximately 120 kDa precursors, they are inserted into membranes of the endoplasmic reticulum (ER) in a hairpin fashion. Both the N-terminal transcription factor domain and the C-terminal regulatory domain face the cytoplasm. These are connected by two transmembrane helices separated by a short loop projecting into the ER lumen. The C-terminal domain of SREBP interacts with the C-terminal domain of SREBP-cleavage-activating protein (SCAP). The N-terminal half of SCAP contains eight transmembrane helices, five of which (helices 2-6) form the sterol-sensing domain. In response to cellular demand for lipid, this complex exits the ER and transits to the Golgi apparatus, where two distinct proteases cleave the SREBP precursor to release the transcriptionally active N-terminus. This process was the first example of regulated intramembrane proteolysis for which the proteases were identified. Recent work has additionally uncovered integral membrane proteins, insig-1 and insig-2, that are required to retain the SREBP-SCAP complex in the ER in the presence of sterols, thus providing a more complete understanding of the control of proteolysis in this complex regulatory pathway.

Other ways to skin a cat: activating SREBP without Scap.

The sterol regulatory element binding protein (SREBP) pathway plays a central role in the global regulation of lipid homeostasis. SREBPs are membrane-bound transcription factors whose proteolytic activation is regulated by cellular lipid levels; when demand for lipid rises, SREBP travels from the endoplasmic reticulum to the Golgi apparatus where it is cleaved by two distinct proteases. Cleavage releases the transcription factor domain of SREBP from the membrane-bound precursor and transcription of its target genes consequently rises. Previously, we isolated Drosophila mutants null for dsrebp and others lacking site-2 protease (ds2p), the second of two Golgi-resident proteases that cleave dSREBP. dScap is a protein needed to escort dSREBP from the ER to the Golgi apparatus. We recently characterized the phenotypes of dscap mutants as well. Here, we describe additional details of phenotypes arising from the inability to activate SREBP appropriately.

Activation of sterol regulatory element binding proteins in the absence of Scap in Drosophila melanogaster.

The escort factor Scap is essential in mammalian cells for regulated activation of sterol regulatory element binding proteins (SREBPs). SREBPs are membrane-bound transcription factors. Cells lacking Scap cannot activate SREBP. They are therefore deficient in the transcription of numerous genes involved in lipid synthesis and uptake; they cannot survive in the absence of exogenous lipid. Here we report that, in contrast to mammalian cells, Drosophila completely lacking dscap are viable. Flies lacking dscap emerge at approximately 70% of the expected rate and readily survive as homozygous stocks. These animals continue to cleave dSREBP in some tissues. Transcription of dSREBP target genes in dscap mutant larvae is reduced compared to wild type. It is greater than in mutants lacking dSREBP and remains responsive to dietary lipids in dscap mutants. Flies lacking dscap do not require the caspase Drice to activate dSREBP. This contrasts with ds2p mutants. ds2p encodes a protease that releases the transcription factor domain of dSREBP from the membrane. Larvae doubly mutant for dscap and ds2p exhibit phenotypes similar to those of ds2p single mutants. Thus, dScap and dS2P, essential components of the SREBP activation machinery in mammalian cells, are dispensable in Drosophila owing to different compensatory mechanisms.

Activation of sterol regulatory element-binding protein by the caspase Drice in Drosophila larvae.

During larval development in Drosophila melanogaster, transcriptional activation of target genes by sterol regulatory element-binding protein (dSREBP) is essential for survival. In all cases studied to date, activation of SREBPs requires sequential proteolysis of the membrane-bound precursor by site-1 protease (S1P) and site-2 protease (S2P). Cleavage by S2P, within the first membrane-spanning helix of SREBP, releases the transcription factor. In contrast to flies lacking dSREBP, flies lacking dS2P are viable. The Drosophila effector caspase Drice cleaves dSREBP, and cleavage requires an Asp residue at position 386, in the cytoplasmic juxtamembrane stalk. The initiator caspase Dronc does not cleave dSREBP, but animals lacking dS2P require both drice and dronc to complete development. They do not require Dcp1, although this effector caspase also can cleave dSREBP in vitro. Cleavage of dSREBP by Drice releases the amino-terminal transcription factor domain of dSREBP to travel to the nucleus where it mediates the increased transcription of target genes needed for lipid synthesis and uptake. Drice-dependent activation of dSREBP explains why flies lacking dS2P are viable, and flies lacking dSREBP itself are not.

Intriguing parasites and intramembrane proteases.

Rhomboid intramembrane proteases occur throughout the kingdoms of life. In this issue of Genes & Development, Baxt and colleagues (pp. 1636-1646) report that the single proteolytic rhomboid (EhROM1) from Entamoeba histolytica cleaves cell surface galactose-binding or N-acetylgalactosamine-binding (Gal/Gal-NAc) lectins. EhROM1 and lectins colocalize during phagocytosis and receptor capping. EhROM1 is found at the base of the cap rather than in the cap proper, suggesting a role in receptor shedding and implying that EhROM1 is crucial for amoebal infection.

Dual roles for cholesterol in mammalian cells.

The structural features of sterols required to support mammalian cell growth have not been fully defined. Here, we use mutant CHO cells that synthesize only small amounts of cholesterol to test the capacity of various sterols to support growth. Sterols with minor modifications of the side chain (e.g., campesterol, beta-sitosterol, and desmosterol) supported long-term growth of mutant cells, but sterols with more complex modifications of the side chain, the sterol nucleus, or the 3-hydroxy group did not. After 60 days in culture, the exogenous sterol comprised >90% of cellular sterols. Inactivation of residual endogenous synthesis with the squalene epoxidase inhibitor NB-598 prevented growth in beta-sitosterol and greatly reduced growth in campesterol. Growth of cells cultured in beta-sitosterol and NB-598 was restored by adding small amounts of cholesterol to the medium. Surprisingly, enantiomeric cholesterol also supported cell growth, even in the presence of NB-598. Thus, sterols fulfill two roles in mammalian cells: (i) a bulk membrane requirement in which phytosterols can substitute for cholesterol and (ii) other processes that specifically require small amounts of cholesterol but are not enantioselective.

The SREBP pathway--insights from Insigs and insects.

Animal cells coordinate lipid homeostasis by end-product feedback regulation of transcription. The control occurs through the proteolytic release of transcriptionally active sterol regulatory element binding proteins (SREBPs) from intracellular membranes. This feedback system has unexpected features that are found in all cells. Here, we consider recently discovered components of the regulatory machinery that govern SREBP processing, as well as studies in Drosophila that indicate an ancient role for the SREBP pathway in integrating membrane composition and lipid biosynthesis.

Mutant mammalian cells as tools to delineate the sterol regulatory element-binding protein pathway for feedback regulation of lipid synthesis.

The tools of somatic cell genetics have been instrumental in unraveling the pathway by which sterol regulatory element-binding proteins (SREBPs) control lipid metabolism in animal cells. SREBPs are membrane-bound transcription factors that enhance the synthesis and uptake of cholesterol and fatty acids. The activities of the SREBPs are controlled by the cholesterol content of cells through feedback inhibition of proteolytic processing. When cells are replete with sterols, SREBPs remain bound to membranes of the endoplasmic reticulum (ER) and are therefore inactive. When cells are depleted of sterols, the SREBPs move to the Golgi complex where two proteases release the active portions of the SREBPs, which then enter the nucleus and activate transcription of target genes. This processing requires three membrane proteins-a sterol-sensing escort protein (SCAP) that transports SREBPs from the ER to the Golgi and two Golgi-located proteases (S1P and S2P) that release SREBPs from membranes. The existence of all three proteins was revealed through analysis of mutant mammalian cells in tissue culture. Their cDNAs and genes were isolated by genetic complementation or by expression cloning. The somatic cell genetic approach described in this article should prove useful for unraveling other complex biochemical pathways in animal cells.

Reconstitution of sterol-regulated endoplasmic reticulum-to-Golgi transport of SREBP-2 in insect cells by co-expression of mammalian SCAP and Insigs.

In mammalian cells, membrane-bound sterol regulatory element-binding proteins (SREBPs) are transported from ER to Golgi where they are processed proteolytically to generate soluble transcription factors that activate lipid synthesis. ER-to-Golgi transport requires SCAP, a sterol-regulated escort protein. In sterol-treated cells, the SCAP/SREBP complex binds to Insig-1 or Insig-2, which retains the complex in the ER, blocking SREBP processing and decreasing lipid synthesis. In Drosophila cells, the endogenous SCAP/SREBP complex is transported to Golgi, but transport is blocked by phosphatidylethanolamine instead of sterols. Here, we show that mammalian SREBP-2 is not transported to Golgi when expressed in Drosophila cells. Transport requires co-expression of mammalian SCAP. Sterols block transport of the mammalian SCAP/SREBP-2 complex, but only when mammalian Insig-1 or -2 is co-expressed. These reconstitution studies define SCAP and Insig as the minimal requirements for sterol-regulated transport of SREBPs from ER to Golgi. They indicate that insect cells can respond to sterols when proper regulatory proteins are expressed.

Three mutations in sterol-sensing domain of SCAP block interaction with insig and render SREBP cleavage insensitive to sterols.

We report the isolation and characterization of a new line of mutant Chinese hamster ovary cells, designated SRD-5, that are resistant to 25HC, a potent suppressor of cleavage of sterol regulatory element-binding proteins (SREBPs) in mammalian cells. In SRD-5 cells, SREBPs are cleaved constitutively, generating transcriptionally active nuclear SREBP even in the presence of sterols. Sequence analysis of SREBP cleavage-activating protein (SCAP) transcripts from SRD-5 cells revealed the presence of a mutation in one SCAP allele that results in substitution of a conserved Leu by Phe at amino acid 315 within the sterol-sensing domain. Sterols fail to inhibit the packaging of SREBPSCAP(L315F) complexes into budding vesicles in vitro. Sterols also fail to induce binding of SCAP(L315F) to insig-1 or insig-2, two proteins that function in the sterol-mediated retention of SREBPSCAP complexes in the endoplasmic reticulum. Similar findings were observed for SCAP(D443N) and SCAP(Y298C), both of which cause a sterol-resistant phenotype. Thus, three different point mutations, each within the sterol-sensing domain of SCAP, prevent sterol-induced binding of SCAP to insig proteins and abolish feedback regulation of SREBP processing by sterols.

Isolation of mutant cells lacking Insig-1 through selection with SR-12813, an agent that stimulates degradation of 3-hydroxy-3-methylglutaryl-coenzyme A reductase.

Insig-1 and Insig-2 are membrane proteins of the endoplasmic reticulum that regulate lipid metabolism by the following two actions: 1) sterol-induced binding to 3-hydroxy-3-methylglutaryl-coenzyme A reductase, an action that leads to ubiquitination and degradation of the enzyme; and 2) sterol-induced binding to SREBP cleavage-activating protein, an action that blocks the proteolytic processing of sterol regulatory element-binding proteins (SREBPs), membrane-bound transcription factors that enhance the synthesis of cholesterol and fatty acids. Here we report the isolation of a new mutant line of Chinese hamster ovary cells, designated SRD-14, in which Insig-1 mRNA and protein are not produced due to a partial deletion of the INSIG-1 gene. The SRD-14 cells were produced by gamma-irradiation, followed by selection with the 1,1-bisphosphonate ester SR-12813, which mimics sterols in accelerating reductase degradation but does not block SREBP processing. SRD-14 cells fail to respond to sterols by promoting reductase ubiquitination and degradation. The rate at which sterols suppress SREBP processing is significantly slower in SRD-14 cells than wild type CHO-7 cells. Sterol regulation of reductase degradation and SREBP processing is restored when SRD-14 cells are transfected with expression plasmids encoding either Insig-1 or Insig-2. These results provide formal genetic proof for the essential role of Insig-1 in feedback control of lipid synthesis in cultured cells.