Lipid moiety of glycosylphosphatidylinositol-anchored proteins contributes to the determination of their final destination in yeast.

Yeasts have two classes of glycosylphosphatidylinositol (GPI)-anchored proteins; one is transferred to the cell wall, whereas the other is retained on the plasma membrane. The lipid moieties of the GPI in Saccharomyces cerevisiae consist of either phosphatidylinositol (PI) or inositolphosphorylceramide (IPC). Cwh43p is involved in the remodeling of lipid from PI to IPC. We found that the GPI lipid moiety of Cwp2p in wild-type cells is PI. To elucidate the physiological role of the lipid remodeling by Cwh43p, we investigated the distribution of Gas1p and Cwp2p by immunoblotting and found that Gas1p with the PI-form GPI lipid moiety in cwh43∆ mutant cells tends to be localized to the cell wall, suggesting that the IPC species in the GPI lipid moiety contributes to the retention of GPI-anchored proteins on the plasma membrane. We also found that CWH43 is genetically related to TED1, which encodes a protein involved in the removal of the ethanolamine phosphate from the second mannose residue in GPI glycan moieties. We propose possible models for the physiological function of Cwh43p and Ted1p in the transfer of GPI-anchored proteins from the plasma membrane to the cell wall.

Identification of a Golgi GPI-N-acetylgalactosamine transferase with tandem transmembrane regions in the catalytic domain.

Many eukaryotic proteins are anchored to the cell surface via the glycolipid glycosylphosphatidylinositol (GPI). Mammalian GPIs have a conserved core but exhibit diverse N-acetylgalactosamine (GalNAc) modifications, which are added via a yet unresolved process. Here we identify the Golgi-resident GPI-GalNAc transferase PGAP4 and show by mass spectrometry that PGAP4 knockout cells lose GPI-GalNAc structures. Furthermore, we demonstrate that PGAP4, in contrast to known Golgi glycosyltransferases, is not a single-pass membrane protein but contains three transmembrane domains, including a tandem transmembrane domain insertion into its glycosyltransferase-A fold as indicated by comparative modeling. Mutational analysis reveals a catalytic site, a DXD-like motif for UDP-GalNAc donor binding, and several residues potentially involved in acceptor binding. We suggest that a juxtamembrane region of PGAP4 accommodates various GPI-anchored proteins, presenting their acceptor residue toward the catalytic center. In summary, we present insights into the structure of PGAP4 and elucidate the initial step of GPI-GalNAc biosynthesis.

A GPI processing phospholipase A2, PGAP6, modulates Nodal signaling in embryos by shedding CRIPTO.

Glycosylphosphatidylinositol-anchored proteins (GPI-APs) can be shed from the cell membrane by GPI cleavage. In this study, we report a novel GPI-processing enzyme, termed post-glycosylphosphatidylinositol attachment to proteins 6 (PGAP6), which is a GPI-specific phospholipase A2 mainly localized at the cell surface. CRIPTO, a GPI-AP, which plays critical roles in early embryonic development by acting as a Nodal coreceptor, is a highly sensitive substrate of PGAP6, whereas CRYPTIC, a close homologue of CRIPTO, is not sensitive. CRIPTO processed by PGAP6 was released as a lysophosphatidylinositol-bearing form, which is further cleaved by phospholipase D. CRIPTO shed by PGAP6 was active as a coreceptor in Nodal signaling, whereas cell-associated CRIPTO activity was reduced when PGAP6 was expressed. Homozygous Pgap6 knockout mice showed defects in early embryonic development, particularly in the formation of the anterior-posterior axis, which are common features with Cripto knockout embryos. These results suggest PGAP6 plays a critical role in Nodal signaling modulation through CRIPTO shedding.

Mutations in PIGY: expanding the phenotype of inherited glycosylphosphatidylinositol deficiencies.

Glycosylphosphatidylinositol (GPI)-anchored proteins are ubiquitously expressed in the human body and are important for various functions at the cell surface. Mutations in many GPI biosynthesis genes have been described to date in patients with multi-system disease and together these constitute a subtype of congenital disorders of glycosylation. We used whole exome sequencing in two families to investigate the genetic basis of disease and used RNA and cellular studies to investigate the functional consequences of sequence variants in the PIGY gene. Two families with different phenotypes had homozygous recessive sequence variants in the GPI biosynthesis gene PIGY. Two sisters with c.137T>C (p.Leu46Pro) PIGY variants had multi-system disease including dysmorphism, seizures, severe developmental delay, cataracts and early death. There were significantly reduced levels of GPI-anchored proteins (CD55 and CD59) on the surface of patient-derived skin fibroblasts (∼20-50% compared with controls). In a second, consanguineous family, two siblings had moderate development delay and microcephaly. A homozygous PIGY promoter variant (c.-540G>A) was detected within a 7.7 Mb region of autozygosity. This variant was predicted to disrupt a SP1 consensus binding site and was shown to be associated with reduced gene expression. Mutations in PIGY can occur in coding and non-coding regions of the gene and cause variable phenotypes. This article contributes to understanding of the range of disease phenotypes and disease genes associated with deficiencies of the GPI-anchor biosynthesis pathway and also serves to highlight the potential importance of analysing variants detected in 5'-UTR regions despite their typically low coverage in exome data.

Glycosylphosphatidylinositol mannosyltransferase II is the rate-limiting enzyme in glycosylphosphatidylinositol biosynthesis under limited dolichol-phosphate mannose availability.

Although the genes involved in the biosynthesis of glycosylphosphatidylinositol (GPI) are well characterized, the regulation of GPI biosynthesis remains unclear. We isolated and characterized a mutant cell line showing decreased surface expression of CD59 and the accumulation of GPI intermediates. The mutant cell line was partially defective in MPDU1, which encodes a protein required for the utilization of dolichol-phosphate mannose. Overexpression of PIGV, which encodes GPI mannosyltransferase II, restored the surface expression of CD59 and normalized the accumulation of GPI intermediates in the mutant cells. Among all known genes involved in GPI biosynthetic pathway, only PIGV had such suppressive activity. PIGV, however, did not restore the abnormality of N-glycosylation caused by MPDU1 mutation. Our results suggest that GPI mannosyltransferase II is the rate-limiting enzyme in GPI biosynthesis under limited dolichol-phosphate mannose availability.

Defective lipid remodeling of GPI anchors in peroxisomal disorders, Zellweger syndrome, and rhizomelic chondrodysplasia punctata.

Many cell surface proteins in mammalian cells are anchored to the plasma membrane via glycosylphosphatidylinositol (GPI). The predominant form of mammalian GPI contains 1-alkyl-2-acyl phosphatidylinositol (PI), which is generated by lipid remodeling from diacyl PI. The conversion of diacyl PI to 1-alkyl-2-acyl PI occurs in the ER at the third intermediate in the GPI biosynthetic pathway. This lipid remodeling requires the alkyl-phospholipid biosynthetic pathway in peroxisome. Indeed, cells defective in dihydroxyacetone phosphate acyltransferase (DHAP-AT) or alkyl-DHAP synthase express only the diacyl form of GPI-anchored proteins. A defect in the alkyl-phospholipid biosynthetic pathway causes a peroxisomal disorder, rhizomelic chondrodysplasia punctata (RCDP), and defective biogenesis of peroxisomes causes Zellweger syndrome, both of which are lethal genetic diseases with multiple clinical phenotypes such as psychomotor defects, mental retardation, and skeletal abnormalities. Here, we report that GPI lipid remodeling is defective in cells from patients with Zellweger syndrome having mutations in the peroxisomal biogenesis factors PEX5, PEX16, and PEX19 and in cells from patients with RCDP types 1, 2, and 3 caused by mutations in PEX7, DHAP-AT, and alkyl-DHAP synthase, respectively. Absence of the 1-alkyl-2-acyl form of GPI-anchored proteins might account for some of the complex phenotypes of these two major peroxisomal disorders.

Mechanism for release of alkaline phosphatase caused by glycosylphosphatidylinositol deficiency in patients with hyperphosphatasia mental retardation syndrome.

Hyperphosphatasia mental retardation syndrome (HPMR), an autosomal recessive disease characterized by mental retardation and elevated serum alkaline phosphatase (ALP) levels, is caused by mutations in the coding region of the phosphatidylinositol glycan anchor biosynthesis, class V (PIGV) gene, the product of which is a mannosyltransferase essential for glycosylphosphatidylinositol (GPI) biosynthesis. Mutations found in four families caused amino acid substitutions A341E, A341V, Q256K, and H385P, which drastically decreased expression of the PIGV protein. Hyperphosphatasia resulted from secretion of ALP, a GPI-anchored protein normally expressed on the cell surface, into serum due to PIGV deficiency. In contrast, a previously reported PIGM deficiency, in which there is a defect in the transfer of the first mannose, does not result in hyperphosphatasia. To provide insights into the mechanism of ALP secretion in HPMR patients, we took advantage of CHO cell mutants that are defective in various steps of GPI biosynthesis. Secretion of ALP requires GPI transamidase, which in normal cells, cleaves the C-terminal GPI attachment signal peptide and replaces it with GPI. The GPI-anchored protein was secreted substantially into medium from PIGV-, PIGB-, and PIGF-deficient CHO cells, in which incomplete GPI bearing mannose was accumulated. In contrast, ALP was degraded in PIGL-, DPM2-, or PIGX-deficient CHO cells, in which incomplete shorter GPIs that lacked mannose were accumulated. Our results suggest that GPI transamidase recognizes incomplete GPI bearing mannose and cleaves a hydrophobic signal peptide, resulting in secretion of soluble ALP. These results explain the molecular mechanism of hyperphosphatasia in HPMR.

Peroxisome dependency of alkyl-containing GPI-anchor biosynthesis in the endoplasmic reticulum.

Glycosylphosphatidylinositol-anchored proteins (GPI-APs) play various roles in cell-cell and cell-environment interactions. GPI is synthesized in the endoplasmic reticulum (ER) from phosphatidylinositol (PI) through step-wise reactions including transfers of monosaccharides and preassembled GPI is transferred en bloc to proteins. Cellular PI contains mostly diacyl glycerol and unsaturated fatty acid in the sn-2 position, whereas mammalian GPI-APs have mainly 1-alkyl-2-acyl PI and almost exclusively stearic acid, a saturated chain, at the sn-2 position. The latter characteristic is the result of fatty acid remodeling occurring in the Golgi, generating GPI-anchors compatible with raft membrane. The former characteristic is the result of diacyl to alkyl-acyl change occurring in the third GPI intermediate, glucosaminyl-inositolacylated-PI (GlcN-acyl-PI). Here we investigated the origin of the sn-1 alkyl-chain in GPI-APs. Using cell lines defective in the peroxisomal alkyl-phospholipid biosynthetic pathway, we demonstrated that generation of alkyl-containing GPI is dependent upon the peroxisomal pathway. We further demonstrated that in cells defective in the peroxisome pathway, the chain composition of the diacyl glycerol moiety in GlcN-acyl-PI is different from those in the first intermediate N-acetylglucosaminyl-PI and cellular PI, indicating that not only diacyl to alkyl-acyl change but also diacyl to diacyl change occurs in GlcN-acyl-PI. We therefore propose a biosynthetic step within GlcN-acyl-PI in which the diacyl glycerol (or diacyl phosphatidic acid) part is replaced by diradyl glycerol (or diradyl phosphatidic acid). These results highlight cooperation of three organelles, the ER, the Golgi, and the peroxisome, in the generation of the lipid portion of GPI-APs.

IL-2 withdrawal induces HTLV-1 expression through p38 activation in ATL cell lines.

Expression of human T-cell leukemia virus type-1 (HTLV-1) in adult T-cell leukemia (ATL) cells is known to be marginal in vivo and inducible in short-term culture. In this study, we demonstrated that withdrawal of interleukin (IL)-2 from IL-2-dependent ATL cell lines resulted in induction of HTLV-1 mRNA and protein expression, and that viral induction was associated with phosphorylation of the stress kinase p38 and its downstream CREB. Pharmacological inhibitors of the p38 pathway suppressed viral expression induced by IL-2 depletion. These results indicate that the stress-induced p38 pathway might up-regulate HTLV-1 gene expression through at least CREB activation.

Augmentation of chemokine production by severe acute respiratory syndrome coronavirus 3a/X1 and 7a/X4 proteins through NF-kappaB activation.

Severe acute respiratory syndrome (SARS) is characterized by rapidly progressing respiratory failure resembling acute/adult respiratory distress syndrome (ARDS) associated with uncontrolled inflammatory responses. Here, we demonstrated that, among five accessory proteins of SARS coronavirus (SARS-CoV) tested, 3a/X1 and 7a/X4 were capable of activating nuclear factor kappa B (NF-kappaB) and c-Jun N-terminal kinase (JNK), and significantly enhanced interleukin 8 (IL-8) promoter activity. Furthermore, 3a/X1 and 7a/X4 expression in A549 cells enhanced production of inflammatory chemokines that were known to be up-regulated in SARS-CoV infection. Our results suggest potential involvement of 3a/X1 and 7a/X4 proteins in the pathological inflammatory responses in SARS.