Regulation of SREBP processing and membrane lipid production by phospholipids in Drosophila.

Animal cells exert exquisite control over the physical and chemical properties of their membranes, but the mechanisms are obscure. We show that phosphatidylethanolamine, the major phospholipid in Drosophila, controls the release of sterol regulatory element-binding protein (SREBP) from Drosophila cell membranes, exerting feedback control on the synthesis of fatty acids and phospholipids. The finding that SREBP processing is controlled by different lipids in mammals and flies (sterols and phosphatidylethanolamine, respectively) suggests that an essential function of SREBP is to monitor cell membrane composition and to adjust lipid synthesis accordingly.

The sterol regulatory element-binding protein pathway: control of lipid homeostasis through regulated intracellular transport.

The sterol regulatory element-binding proteins (SREBPs) are membrane-bound transcription factors that play a central role in cellular lipid homeostasis through the end-product feedback regulation of lipid synthesis. This feedback pathway is best understood in the case of cholesterol. Accumulation of cholesterol suppresses the proteolytic release of the transcriptionally active amino-terminal fragment of SREBP from the membrane-bound precursor. Experiments reported during the past year have led to a more complete understanding of the mechanisms that regulate the processing of SREBPs and their role in cellular lipid homeostasis. Regulation of lipid homeostasis is intimately associated with intracellular membrane trafficking; SREBPs undergo regulated transport from the endoplasmic reticulum to the Golgi apparatus in response to cellular lipid demand. The regulated step in this transport is the budding of a complex of SREBP and SREBP cleavage-activating protein into vesicles. In the present review we focus on recent results that give a more detailed picture of the mechanisms that are involved in end-product feedback regulation of lipid homeostasis.

Scanning electron microscopic analysis of skin resolution as an aid in identifying trauma in forensic investigations.

The forensic investigator is frequently confronted with cases that present with wounds and blunt force trauma. Presently, the forensic investigator depends upon previous experience and further investigative deduction of the crime scene to analyze these injuries. Although not readily apparent to the naked eye, many skin tissue injuries can be visualized with scanning electron microscopy (SEM). This study was designed to establish skin trauma resolution using SEM in various skin preparations. Tissue trauma was induced on leather, preserved skin, fresh skin, and living skin using dies of varying thread size. Calibrated pressure forces in pounds per square inch (psi) were applied and impressions made using vinyl polysiloxane. Positive replicas of the tissues were prepared for SEM using isocyanate resin. After sputter coating the cast with 35 nm of gold-palladium, electron micrographs were generated using a Jeol JSM-5310LV scanning electron microscope. To establish resolution, thread widths of 52, 104, and 208 threads per inch (tpi) and trauma forces of 150, 200, and 250 psi were used to produce the impressions. Microgrooves that were identified on the die threads were analyzed. The optimum pressure for resolution studies was 150 psi using the 52 tpi die on the leather sample (4.67 +/- 0.88 microm, p = 0.046 and 0.025, respectively, by ANOVA). The resolution was compared to that of leather using preserved, fresh, and living skin. The resolution in preserved and fresh skin was less than for leather (9.00 +/- 1.73 and 10.5 +/- 4.5 versus 4.67 +/- 0.88 microm, p = 0.09 and p = 0.20, respectively). Living skin resolution was 3 microm at 52 tpi and 100 psi. Various implements of blunt force trauma were also examined using the leather sample. Time after trauma resolution was examined at 0 (3 microm), 5 (6 microm), 10 (8 microm), and 20 (9 microm) min in living tissue. A comparison between the microgrooves on the die replicas and the tissue trauma impressions revealed striking agreement for both linearity and resolution. Analysis of the microgrooves suggests that discrete morphological characteristics are seen in skin tissue traumas. This method could expand the tools available for the forensic investigation of blunt force trauma.

ER stress induces cleavage of membrane-bound ATF6 by the same proteases that process SREBPs.

ATF6 is a membrane-bound transcription factor that activates genes in the endoplasmic reticulum (ER) stress response. When unfolded proteins accumulate in the ER, ATF6 is cleaved to release its cytoplasmic domain, which enters the nucleus. Here, we show that ATF6 is processed by Site-1 protease (S1P) and Site-2 protease (S2P), the enzymes that process SREBPs in response to cholesterol deprivation. ATF6 processing was blocked completely in cells lacking S2P and partially in cells lacking S1P. ATF6 processing required the RxxL and asparagine/proline motifs, known requirements for S1P and S2P processing, respectively. Cells lacking S2P failed to induce GRP78, an ATF6 target, in response to ER stress. ATF6 processing did not require SCAP, which is essential for SREBP processing. We conclude that S1P and S2P are required for the ER stress response as well as for lipid synthesis.

Failure to cleave sterol regulatory element-binding proteins (SREBPs) causes cholesterol auxotrophy in Chinese hamster ovary cells with genetic absence of SREBP cleavage-activating protein.

We describe a line of mutant Chinese hamster ovary cells, designated SRD-13A, that cannot cleave sterol regulatory element-binding proteins (SREBPs) at site 1, due to mutations in the gene encoding SREBP cleavage-activating protein (SCAP). The SRD-13A cells were obtained by two rounds of gamma-irradiation followed first by selection for a deficiency of low density lipoprotein receptors and second for cholesterol auxotrophy. In the SRD-13A cells, the only detectable SCAP allele encodes a truncated nonfunctional protein. In the absence of SCAP, the site 1 protease fails to cleave SREBPs, and their transcriptionally active NH(2)-terminal fragments cannot enter the nucleus. As a result, the cells manifest a marked reduction in the synthesis of cholesterol and its uptake from low density lipoproteins. The SRD-13A cells grow only when cholesterol is added to the culture medium. SREBP cleavage is restored and the cholesterol requirement is abolished when SRD-13A cells are transfected with expression vectors encoding SCAP. These results provide formal proof that SCAP is essential for the cleavage of SREBPs at site 1.

Membrane topology of S2P, a protein required for intramembranous cleavage of sterol regulatory element-binding proteins.

In sterol-depleted mammalian cells, a two-step proteolytic process releases the NH(2)-terminal domains of sterol regulatory element-binding proteins (SREBPs) from membranes of the endoplasmic reticulum (ER). These domains translocate into the nucleus, where they activate genes of cholesterol and fatty acid biosynthesis. The SREBPs are oriented in the membrane in a hairpin fashion, with the NH(2)- and COOH-terminal domains facing the cytosol and a single hydrophilic loop projecting into the lumen. The first cleavage occurs at Site-1 within the ER lumen to generate an intermediate that is subsequently released from the membrane by cleavage at Site-2, which lies within the first transmembrane domain. A membrane protein, designated S2P, a putative zinc metalloprotease, is required for this cleavage. Here, we use protease protection and glycosylation site mapping to define the topology of S2P in ER membranes. Both the NH(2) and COOH termini of S2P face the cytosol. Most of S2P is hydrophobic and appears to be buried in the membrane. All three of the long hydrophilic sequences of S2P can be glycosylated, indicating that they all project into the lumen. The HEIGH sequence of S2P, which contains two potential zinc-coordinating residues, is contained within a long hydrophobic segment. Aspartic acid 467, located approximately 300 residues away from the HEIGH sequence, appears to provide the third coordinating residue for the active site zinc. This residue, too, is located in a hydrophobic sequence. The hydrophobicity of these sequences suggests that the active site of S2P is located within the membrane in an ideal position to cleave its target, a Leu-Cys bond in the first transmembrane helix of SREBPs.

Molecular identification of the sterol-regulated luminal protease that cleaves SREBPs and controls lipid composition of animal cells.

The lipid composition of animal cells is controlled by SREBPs, transcription factors released from membranes by sterol-regulated proteolysis. Release is initiated by Site-1 protease (S1P), which cleaves SREBPs in the ER luminal loop between two membrane-spanning regions. To clone S1P, we prepared pCMV-PLAP-BP2, which encodes a fusion protein that contains placental alkaline phosphatase (PLAP) in the ER lumen flanked by cleavage sites for signal peptidase and S1P. In sterol-deprived cells, cleavage by both proteases leads to PLAP secretion. PLAP is not secreted by SRD-12B cells, cholesterol auxotrophs that lack S1P. We transfected SRD-12B cells with pCMV-PLAP-BP2 plus pools of CHO cDNAs and identified a cDNA that restores Site-1 cleavage and PLAP secretion. The cDNA encodes S1P, an intraluminal 1052-amino-acid membrane-bound subtilisin-like protease. We propose that S1P is the sterol-regulated protease that controls lipid metabolism in animal cells.

Isolation of cholesterol-requiring mutant Chinese hamster ovary cells with defects in cleavage of sterol regulatory element-binding proteins at site 1.

The synthesis and uptake of cholesterol requires transcription factors designated sterol regulatory element-binding proteins (SREBPs). SREBPs are bound to membranes in a hairpin orientation with their transcriptionally active NH2-terminal segments facing the cytosol. The NH2-terminal segments are released from membranes by two-step proteolysis initiated by site 1 protease (S1P), which cleaves in the luminal loop between two membrane-spanning segments. Next, site 2 protease (S2P) releases the NH2-terminal fragment of SREBP. The S2P gene was recently isolated by complementation cloning using Chinese hamster ovary cells that require cholesterol for growth, due to a mutation in the S2P gene. A similar approach cannot be used for S1P because all previous cholesterol auxotrophs manifest defects in S2P, which is encoded by a single copy gene. To circumvent this problem, in the current studies we transfected Chinese hamster ovary cells with the S2P cDNA, assuring multiple copies. We mutagenized the cells, selected for cholesterol auxotrophy, and identified two mutant cell lines (SRD-12A and -12B) that fail to cleave SREBPs at site 1. Complementation analysis demonstrated that the defects in both cell lines are recessive and noncomplementing, indicating a mutation in the same gene. These cells should now be useful for expression cloning of the sterol-regulated S1P gene.

UbcD1, a Drosophila ubiquitin-conjugating enzyme required for proper telomere behavior.

The end-to-end association of chromosomes through their telomeres has been observed in normal cells of certain organisms, as well as in senescent and tumor cells. The molecular mechanisms underlying this phenomenon are currently unknown. We show here that five independent mutant alleles in the Drosophila UbcD1 gene cause frequent telomere-telomere attachments during both mitosis and male meiosis that are not seen in wild type. These telomeric associations involve all the telomeres of the D. melanogaster chromosome complement, albeit with different frequencies. The pattern of telomeric associations observed in UbcD1 mutants suggests strongly that the interphase chromosomes of wild-type larval brain cells maintain a Rab1 orientation within the nucleus, with the telomeres and centromeres segregated to opposite sides of the nucleus. The UbcD1 gene encodes a class I ubiquitin-conjugating (E2) enzyme. This indicates that ubiquitin-mediated proteolysis is normally needed to ensure proper telomere behavior during Drosophila cell division. We therefore suggest that at least one of the targets of UbcD1 ubiquitination is a telomere-associated polypeptide that may help maintain proper chromosomal orientation during interphase.

Complementation cloning of S2P, a gene encoding a putative metalloprotease required for intramembrane cleavage of SREBPs.

We report the cloning of a gene, S2P, that encodes a putative metalloprotease required for intramembrane proteolysis of sterol-regulatory element-binding proteins (SREBPs) at Site-2. SREBPs are membrane-bound transcription factors that activate genes regulating cholesterol metabolism. The active NH2-terminal domains of SREBPs are released from membranes by sequential cleavage at two sites: Site-1, within the lumen of the endoplasmic reticulum; and Site-2, within a transmembrane segment. The human S2P gene was cloned by complementation of mutant CHO cells that cannot cleave SREBPs at Site-2 and are cholesterol auxotrophs. S2P defines a new family of polytopic membrane proteins that contain an HEXXH sequence characteristic of zinc metalloproteases. Mutation of the putative zinc-binding residues abolishes S2P activity. S2P encodes an unusual metalloprotease that cleaves proteins within transmembrane segments.

cDNA cloning and chromosomal localization of the genes encoding the alpha- and beta-subunits of human Rab geranylgeranyl transferase: the 3' end of the alpha-subunit gene overlaps with the transglutaminase 1 gene promoter.

We have isolated and sequenced the complete coding sequences of the human genes for the alpha- and beta-subunits of Rab geranylgeranyl transferase (Rab GGTase). The alpha- and beta-subunit genes code for proteins of 567 and 331 amino acids, respectively, showing 91 and 95% amino acid identity to their rat counterparts. We employed fluorescence in situ hybridization to map the beta-subunit gene to human chromosome 1p31. The alpha-subunit gene could be assigned to 14q11.2, less than 2 kb upstream of the transcription initiation site of the gene for transglutaminase 1 (TGM1). The two genes are arranged in tandem in a head-to-tail orientation. The short intergenic sequence between the two loci contains several promoter elements that are involved in the induction of TGM1 gene expression in squamous cells. These results suggest that cis-acting factors for cell-type-specific transcription of one gene are located within the transcribed region of a functionally unrelated gene.

Sterol-regulated release of SREBP-2 from cell membranes requires two sequential cleavages, one within a transmembrane segment.

Sterol regulatory element binding proteins (SREBPs) are transcription factors attached to the endoplasmic reticulum. The NH2-segment, which activates transcription, is connected to membranes by a hairpin anchor formed by two transmembrane sequences and a short lumenal loop. Using H-Ras-SREBP-2 fusion proteins, we show that the NH2-segment is released from membranes by two sequential cleavages. The first, regulated by sterols, occurs in the lumenal loop. The second, not regulated by sterols, occurs within the first transmembrane domain. The liberated NH2-segment enters the nucleus and activates genes controlling cholesterol synthesis and uptake. Certain mutant Chinese hamster ovary cells are auxotrophic for cholesterol because they fail to carry out the second cleavage; the NH2-segment remains membrane-bound and transcription is not activated.