Expression of LASS2 controlled by LAG1 or ADH1 promoters cannot functionally complement Lag1p

LAG1 contributes to the substrate specificity and catalytic activity of ceramide synthases in Saccharomyces cerevisiae. Double deletion of LAG1 and its yeast homologue LAC1 results in the slow growth defect of the cell under certain genetic backgrounds. LASS2, containing the conserved TLC domain and the specific HOX domain, is a human homologue of Lag1p. In this study, shuffling tests and tetrad analyses were carried out to examine the complementation between Lag1p and LASS2 or its fragment containing the TLC domain but lacking the HOX domain (LASS2DeltaHOX). Controlled by either the natural weak LAG1 promoter or the strong yeast ADH1 promoter, LASS2 and LASS2DeltaHOX could not rescue the slow growth defect of double mutant. The results indicated that LASS2 or LASS2DeltaHOX could not functionally complement Lag1p.     

膜蛋白LASS2的表达研究

利用多种原核表达系统、真核体外翻译系统和细菌/杆状病毒(Bac to Bac)的昆虫表达系统对一个具有重要生理功能的人的膜蛋白LASS2(Homo sapienslongevity assurance homologue 2 of yeastLAG1)进行表达研究。在原核表达系统中仅能够表达其羧基端胞外区片段却不能表达完整的LASS2蛋白,并制备了该片段的抗体。完整的LASS2蛋白能够在两种真核表达系统中进行表达,SDS-PAGE分析结果表明,表达产物分子量为约28kD的LASS蛋白, Western印迹分析也证实了这一结果, 并利用Ni-NTA树脂亲和层析将该蛋白纯化,纯度达到90%以上。     

Silencing of a novel tumor metastasis suppressor gene LASS2/TMSG1 promotes invasion of prostate cancer cell in vitro through increase of vacuolar ATPase activity

Homo sapiens longevity assurance homologue 2 of yeast LAG1 (LASS2), also known as tumor metastasis suppressor gene 1 (TMSG1), is a newly found tumor metastasis suppressor gene in 1999. Preliminary studies showed that it not only suppressed tumor growth but also closely related to tumor metastasis, however, its molecular mechanisms is still unclear. There have been reported that protein encoded by LASS2/TMSG-1 could directly interact with the C subunit of Vacuolar ATPase (V-ATPase), which suggested that LASS2/TMSG1 might inhibit the invasion and metastasis through regulating the function of V-ATPase. Thus, in this study, we explored the effect of small interference RNA (siRNA) targeting LASS2/TMSG1 on the invasion of human prostate carcinoma cell line PC-3M-2B4 and its molecular mechanisms associated with the V-ATPase. Real-time fluorogentic quantitative PCR (RFQ-PCR) and Western blot revealed dramatic reduction of 84.5% and 60% in the levels of LASS2/TMSG1 mRNA and protein after transfection of siRNA in PC-3M-2B4 cells. The V-ATPase activity and extracellular hydrogen ion concentration were significantly increased in 2B4 cells transfected with the LASS2/TMSG1-siRNA compared with the controls. The activity of secreted MMP-2 was up-regulated in LASS2/TMSG1-siRNA treated cells compared with the controls; and the capacity for migration and invasion in LASS2/TMSG1-siRNA treated cells was significantly higher than the controls. Thus, we concluded that silencing of LASS2/TMSG1 may promote invasion of prostate cancer cell in vitro through increase of V-ATPase activity and extracellular hydrogen ion concentration and in turn the activation of secreted MMP-2.     

High expression of LASS2 is associated with unfavorable prognosis in patients with ovarian cancer

Homo sapiens longevity assurance homolog 2 of yeast LAG1 (LASS2), is a gene isolated from a human liver complementary DNA library. In this study, we found that LASS2 protein level was positively related to International Federation of Gynecology and Obstetrics (FIGO) stage and LASS2-negative tumors showed significant association with longer disease-free survival (DFS) and overall survival (OS) in ovarian cancer patients. The heterogeneous expression of LASS2 had been exhibited in diverse ovarian cancer cells. A significantly lower messenger RNA (mRNA) and protein level of LASS2 was seen in 3AO cell compared with those in other types of ovarian cancer cells. Meanwhile, the mRNA and protein levels of LASS2 in ES-2 and NIH:OVCAR-3 cells were obviously higher. LASS2 overexpression in 3AO cell could promote migration, invasion, and metastasis abilities in vitro and in vivo, while LASS2 knockdown in ES-2 and NIH:OVCAR-3 cells had the opposite effects. The oncogenic capacity of LASS2 in ovarian cancer may be mediated by increased expression of YAP/TAZ. It is indicated that lowering the expression of LASS2 is likely to serve as an unprecedented approach for the treatment of ovarian cancer.     

Necessary role for the Lag1p motif in (dihydro)ceramide synthase activity

Lag1 (longevity assurance gene 1) homologues, a family of transmembrane proteins found in all eukaryotes, have been shown to be necessary for (dihydro)ceramide synthesis. All Lag1 homologues contain a highly conserved stretch of 52 amino acids known as the Lag1p motif. However, the functional significance of the conserved Lag1p motif for (dihydro)ceramide synthesis is currently unknown. In this work, we have investigated the function of the motif by introducing eight point mutations in the Lag1p motif of the mouse LASS1 (longevity assurance homologue 1 of yeast Lag1). The (dihydro)ceramide synthase activity of the mutants was tested using microsomes in HeLa cells and in vitro. Six of the mutations resulted in loss of activity in cells and in vitro. In addition, our results showed that C18:0 fatty acid CoA (but not cis-C18:1 fatty acid CoAs) are substrates for LASS1 and that LASS1 in HeLa cells is sensitive to fumonisin B1, an in vitro inhibitor of (dihydro)ceramide synthase. Moreover, we mutated the Lag1p motif of another Lag homologue, human LASS5. The amino acid substitutions in the human LASS5 were the same as in mouse LASS1, and had the same effect on the in vitro activity of LASS5, suggesting the Lag1p motif appears to be essential for the enzyme activity of all Lag1 homologues.     

Identification of novel splice variants of the human catalytic subunit Cbeta of cAMP-dependent protein kinase.

Four different isoforms of the catalytic subunit of cAMP-dependent protein kinase, termed Calpha, Cbeta, Cgamma and PrKX have been identified. Here we demonstrate that the human Cbeta gene encodes six splice variants, designated Cbeta1, Cbeta2, Cbeta3, Cbeta4, Cbeta4ab and Cbeta4abc. The Cbeta splice variants differ in their N-terminal ends due to differential splicing of four different forms of exon 1 designated exon 1-1, 1-2, 1-3, 1-4 and three exons designated a, b and c. All these exons are located upstream of exon 2 in the Cbeta gene. The previously identified human Cbeta variant has been termed Cbeta1, and is similar to the Cbeta isoform identified in the mouse, ox, pig and several other mammals. Human Cbeta2, which is the homologue of bovine Cbeta2, has no homologue in the mouse. Human Cbeta3 and Cbeta4 are homologous to the murine Cbeta3 and Cbeta2 splice variants, whereas human Cbeta4ab and Cbeta4abc represent novel isofoms previously not identified in any other species. At the mRNA level, the Cbeta splice variants reveal tissue specific expression. Cbeta1 was most abundantly expressed in the brain, with low-level expression in several other tissues. The Cbeta3 and Cbeta4 splice variants were uniquely expressed in human brain in contrast to Cbeta2, which was most abundantly expressed in tissues of the immune system, with no detectable expression in brain. We suggest that the various Cbeta splice variants when complexed with regulatory subunits may give rise to novel holoenzymes of protein kinase A that may be important for mediating specific effects of cAMP.     

D,L-Threo-1-phenyl-2-decanoylamino-3-morpholino-1-propanol (DL-PDMP) increases endoplasmic reticulum stress, autophagy and apoptosis accompanying ceramide accumulation via ceramide synthase 5 protein expression in A549 cells

In A549 cells, the addition of D,L-threo-1-phenyl-2-decanoylamino-3-morpholino-1-propanol (DL-PDMP) led to marked autophagy with massive microtubule-associated protein 1 light chain 3B (LC3B)-II protein expression as an indication of autophagy and a steep decrease of p62 protein as a co-indication of autophagy. The addition of DL-PDMP caused massive autophagy with an increase of CAAT/enhancer binding protein homologous protein (CHOP) expression as the marker of endoplasmic reticulum (ER) stress, lactate dehydrogenase (LDH) release without caspase 3 activation and many autophagic vacuoles/devoid of a cell membrane on morphology. On the other hand, the addition of DL-PDMP caused an increase in cellular or subcellular ceramides (Cers), especially palmitoyl-Cer, based on de novo synthesis of Cer, and led to caspase-independent apoptosis. Marked increases of Cer levels in the nuclear envelope were observed 17 h after the addition. The elevations of Cer synthase activity and longevity-assurance homologue (LASS)5 protein expression were observed in subcellular fractions from 30 min until 2 h after the addition. However, the elevations of Cer synthase activity were independent of reactive oxygen species generation or cytochrome P450 4F2 activity. Since an increase in LASS5 protein expression in subcellular fraction occur in preference to the variation of LC3B-II protein expression via CHOP expression after the addition and Cer accumulation induced by the addition contributes to ER stress, it is thought that an elevation of Cer synthase activity via LASS5 protein expression associate to autophagy via CHOP expression (ER stress) with the addition.     

Molecular cloning and analysis of the mouse homologue of the tumor-associated mucin, MUC1, reveals conservation of potential O-glycosylation sites, transmembrane, and cytoplasmic domains and a loss of minisatellite-like polymorphism.

We present here the full-length cDNA sequence and genomic structure of the mouse homologue of the tumor-associated mucin, MUC1. This mucin (previously called polymorphic epithelial mucin) is present at the apical surface of most glandular epithelial cells. The mouse gene, Muc-1, encodes an integral membrane protein with 40% of its coding capacity made up of serine, threonine, and proline, a composition typical of a highly O-glycosylated protein. The mucin core protein consists of an amino-terminal signal sequence, a tandem repeat domain encoding 16 repeats of 20-21 amino acids, and unique sequence containing transmembrane and cytoplasmic domains. Homology with the human protein is only 34% in the tandem repeat domain, mainly showing conservation of serines and threonines, presumed sites of O-linked carbohydrate attachment. Homology rises to 87% in the transmembrane and cytoplasmic domains, suggesting that these regions may be functionally important. The pattern of expression of the mouse mucin is very similar to that of its human counterpart and accordingly the two promoter regions share high homology, 74%, although previously identified potential hormone-responsive elements are not conserved. Interestingly, the mouse homologue, unlike its human counterpart does not exhibit a variable number tandem repeat polymorphism. We present evidence that suggests that the mouse gene was at one time polymorphic but has mutated away from this state.     

Coding sequence, genomic organization, chromosomal localization, and expression pattern of the signalosome component Cops2: the mouse homologue of Drosophila alien

The Drosophila alien gene is highly homologous to the human thyroid receptor interacting protein, TRIP15/COPS2, which is a component of the recently identified signalosome protein complex. We identified the mouse homologue of Drosophila alien through homology searches of the EST database. We found that the mouse cDNA encodes a predicted 443-amino-acid protein, which migrates at approximately 50 kDa. The gene for the mouse alien homologue, named Cops2, includes 12 coding exons spanning approximately 30 kb of genomic DNA on the central portion of mouse chromosome 2. Mouse Cops2 is widely expressed in embryonic, fetal, and adult tissues beginning as early as E7.5. Mouse Cops2 cDNA hybridizes to two mRNA bands in all tissues at approximately 2.3 and approximately 4 kb, with an additional approximately 1.9-kb band in liver. Immunostaining of native and epitope tagged proteins localized the mouse Cops2 protein in both the cytoplasm and the nucleus, with larger amounts in the nucleus in some cells.     

Cloning, chromosome mapping and functional characterization of a human homologue of murine gtse-1 (B99) gene

Murine Gtse-1 (G(2) and S phase expressed protein), previously named B99, is a wt-p53 inducible gene that encodes a microtubule-localized protein which is able to induce G(2)/M phase accumulation when ectopically expressed. Here we report the cloning and characterization of a new cDNA (GTSE-1) encoding a human homologue of the mouse Gtse-1 protein. Chromosome mapping of mouse and human genes assigned Gtse-1 to chromosome 15 and GTSE-1 to chromosome 22q13.2-q13.3 in a region with conserved synteny to that where Gtse-1 mapped. Analysis of the genomic structure revealed that GTSE-1 contains at least 11 exons and 10 introns, spanning approximately 33kb of genomic DNA. Similar to murine Gtse-1, the product of GTSE-1 localized to the microtubules, was able to delay G(2)/M progression when ectopically expressed and was cell cycle regulated. Taken together, these results indicate GTSE-1 as the human functional homologue of murine Gtse-1.     

LASS5 is a bona fide dihydroceramide synthase that selectively utilizes palmitoyl-CoA as acyl donor

We demonstrated recently (Riebeling, C., Allegood, J.C., Wang, E., Merrill, A. H. Jr., and Futerman, A. H. (2003) J. Biol. Chem. 278, 43452-43459) that upon over-expression in human embryonic kidney cells, longevity assurance gene homolog 5 (LASS5, previously named TRH4) elevates the synthesis of (dihydro)ceramides selectively enriched in palmitic acid. To determine whether LASS5 is a bona fide dihydroceramide synthase or, alternatively, whether it modifies an endogenous dihydroceramide synthase, we over-expressed LASS5 with a hemagglutinin (HA) tag at the C terminus, solubilized it using digitonin, and purified it by immunoprecipitation. Solubilized LASS5-HA displays the same fatty acid selectivity as the membrane-bound enzyme. After elution from agarose beads, only one band could be detected by SDS-PAGE, and its identity was confirmed to be LASS5 by mass spectrometry. Dihydroceramide synthase activity of the eluted LASS5-HA protein was totally dependent on exogenously added phospholipids. Moreover, eluted LASS5-HA was highly selective toward palmitoyl-CoA as acyl donor and was inhibited by the (dihydro)ceramide synthase inhibitor, fumonisin B1. This study identifies LASS5 as a genuine dihydroceramide synthase and demonstrates that mammalian dihydroceramide synthases do not require additional subunits for their activity.     

Human homologues of LAG1 reconstitute Acyl-CoA-dependent ceramide synthesis in yeast

Lag1p and Lac1p are two highly homologous membrane proteins of the endoplasmic reticulum. lag1delta lac1delta double mutants in Saccharomyces cerevisiae lack an acyl-CoA-dependent ceramide synthase and are either very sick or nonviable, depending on the genetic background. LAG1 and LAC1 are members of a large eukaryotic gene family that shares the Lag1 motif, and some members of this family additionally contain a DNA-binding HOX homeodomain. Here we show that several human LAG1 homologues can rescue the viability of lag1delta lac1delta yeast cells and restore acyl-CoA-dependent ceramide and sphingolipid biosynthesis. When tested in a microsomal assay, Lac1p and Lag1p had a strong preference for C26:0-CoA over C24:0-CoA, C20-CoA, and C16-CoA, whereas some human homologues preferred C24:0-CoA and CoA derivatives with shorter fatty acids. This suggests that LAG1 proteins are related to substrate recognition and to the catalytic activity of ceramide synthase enzymes. CLN8, another human LAG1 homologue implicated in ceroid lipofuscinosis, could not restore viability to lag1delta lac1delta yeast mutants.     

Human mitochondrial pyrophosphatase: cDNA cloning and analysis of the gene in patients with mtDNA depletion syndromes

Pyrophosphatases (PPases) catalyze the hydrolysis of inorganic pyrophosphate generated in several cellular enzymatic reactions. A novel human pyrophosphatase cDNA encoding a 334-amino-acid protein approximately 60% identical to the previously identified human cytosolic PPase was cloned and characterized. The novel enzyme, named PPase-2, was enzymatically active and catalyzed hydrolysis of pyrophosphate at a rate similar to that of the previously identified PPase-1. A functional mitochondrial import signal sequence was identified in the N-terminus of PPase-2, which targeted the enzyme to the mitochondrial matrix. The human pyrophosphatase 2 gene (PPase-2) was mapped to chromosome 4q25 and the 1.4-kb mRNA was ubiquitously expressed in human tissues, with highest levels in muscle, liver, and kidney. The yeast homologue of the mitochondrial PPase-2 is required for mitochondrial DNA maintenance and yeast cells lacking the enzyme exhibit mitochondrial DNA depletion. We sequenced the PPA2 gene in 13 patients with mitochondrial DNA depletion syndromes (MDS) of unknown cause to determine if mutations in the PPA2 gene of these patients were associated with this disease. No pathogenic mutations were identified in the PPA2 gene of these patients and we found no evidence that PPA2 gene mutations are a common cause of MDS in humans.     

Cloning, mapping and tissue-specific expression of Drosophila clathrin-associated protein AP50 gene.

The Drosophila homologue of AP50, the medium chain of clathrin-associated protein complex AP-2, was identified and characterized from the Drosophila Expressed Sequence Tag database. The Drosophila AP50 is 86% identical to that of mouse and human, and 80% identical to the Caenorhabditis elegans homologue. It is a single-copy gene with two mini-introns in the coding region and it maps to position 94B1-B2 on polytene chromosomes. Two P1 clones, DS01102 and DS0104, were identified that contain the AP50 gene. Alternative 5' UTR splicing is involved in the regulation of AP50 expression. AP50 expression is highly enriched in the central nervous system and midgut caecum during embryo development, and its function is discussed. The two other Drosophila members of the medium-chain family of clathrin-associated protein complexes, AP47 and mu3, have also been identified and mapped to 85D20-D27 and 6E1-E4, respectively.     

CASP,the alternatively spliced product of the gene encoding the CCAAT-displacement protein transcription factor,is a Golgi membrane protein related to giantin

Large coiled-coil proteins are being found in increasing numbers on the membranes of the Golgi apparatus and have been proposed to function in tethering of transport vesicles and in the organization of the Golgi stack. Members of one class of Golgi coiled-coil protein, comprising giantin and golgin-84, are anchored to the bilayer by a single C-terminal transmembrane domain (TMD). In this article, we report the characterization of another mammalian coiled-coil protein, CASP, that was originally identified as an alternatively spliced product of the CUTL1 gene that encodes CCAAT-displacement protein (CDP), the human homologue of the Drosophila homeodomain protein Cut. We find that the Caenorhabditis elegans homologues of CDP and CASP are also generated from a single gene. CASP lacks the DNA binding motifs of CDP and was previously reported to be a nuclear protein. Herein, we show that it is in fact a Golgi protein with a C-terminal TMD and shares with giantin and golgin-84 a conserved histidine in its TMD. However, unlike these proteins, CASP has a homologue in Saccharomyces cerevisiae, which we call COY1. Deletion of COY1 does not affect viability, but strikingly restores normal growth to cells lacking the Golgi soluble N-ethylmaleimide-sensitive factor attachment protein receptor Gos1p. The conserved histidine is necessary for Coy1p's activity in cells lacking Gos1p, suggesting that the TMD of these transmembrane Golgi coiled-coil proteins is directly involved in their function.