phosphatidylinositol glycan anchor biosynthesis, class A OKDB#: 1775
 Symbols: PIGA Species: human

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General Comment Many cell surface proteins are anchored to the membrane by a glycolipid termed the glycosylphosphatidylinositol (GPI) anchor, which is covalently attached to the C-terminus of the peptide. The core of the GPI anchor consists of multiple components and its biosynthesis involves multiple genes, such as those for glycosyltransferases. By expression cloning methods, Miyata et al. (1993) cloned a gene they termed PIGA (for phosphatidylinositol glycan class A) which takes part in the synthesis of N-acetylglucosaminyl phosphatidylinositol (GlcNAc-PI), the first intermediate in the biosynthetic pathway of GPI anchor.

NCBI Summary: This gene encodes a protein required for synthesis of N-acetylglucosaminyl phosphatidylinositol (GlcNAc-PI), the first intermediate in the biosynthetic pathway of GPI anchor. The GPI anchor is a glycolipid found on many blood cells and which serves to anchor proteins to the cell surface. Paroxysmal nocturnal hemoglobinuria, an acquired hematologic disorder, has been shown to result from mutations in this gene. Alternate splice variants have been characterized. A related pseudogene is located on chromosome 12. [provided by RefSeq, Jun 2010]
General function Enzyme
Cellular localization Cytoplasmic, Plasma membrane
Ovarian function Early embryo development
Comment GPI-anchor Synthesis Is Indispensable for the Germline Development of the Nematode Caenorhabditis elegans. Murata D et al. Glycosylphosphatidylinositol (GPI)-anchor attachment is one of the most common post-translational protein modifications. Using the nematode Caenorhabditis elegans, we determined that GPI-anchored proteins are present in germline cells and distal tip cells (DTCs), which are essential for the maintenance of the germline stem cell niche. We identified 24 C. elegans genes involved in GPI-anchor synthesis. Inhibition of various steps of GPI-anchor synthesis by RNAi or gene knockout resulted in abnormal development of oocytes and early embryos, and both lethal and sterile phenotypes were observed. The piga-1 gene (ortholog of human PIGA) codes for the catalytic subunit of the phosphatidylinositol N-acetylglucosaminyltransferase complex, which catalyzes the first step of GPI-anchor synthesis. We isolated piga-1 knockout worms and found that GPI-anchor synthesis is indispensable for the maintenance of mitotic germline cell number. The knockout worms displayed 100% lethality with decreased mitotic germline cells and abnormal eggshell formation. Using cell-specific rescue of the null allele, we showed that expression of piga-1 in somatic gonads and/or in germline is sufficient for normal embryonic development and the maintenance of the germline mitotic cells. These results clearly demonstrate that GPI-anchor synthesis is indispensable for germline formation and for normal development of oocytes and eggs.
Expression regulated by
Ovarian localization Oocyte
Follicle stages Antral

Species: mouse -
type: null mutation
fertility: infertile - ovarian defect
Comment: Alfieri JA, et al reported infertility in female mice with an oocyte-specific knockout of GPI-anchored proteins. Glycosylphosphatidylinositol-anchored proteins on the egg surface have been proposed to play a role in gamete fusion on the basis of in vitro experiments. Oocyte-specific knockout mice were created using the Cre/loxP system to delete a portion of the Pig-a gene, which encodes an enzyme involved in GPI anchor biosynthesis. Conditional Pig-a-knockout females are infertile, and eggs recovered from the females after mating are unfertilized. In in vitro assays, the knockout eggs are severely deficient in their ability to fuse with sperm. These results demonstrate that GPI-anchored proteins are required for gamete fusion. Loss of the GPI-anchored complement of plasma membrane proteins could prevent fusion by altering the organization and function of GPI-anchored protein-containing lipid domains. Alternatively, a single GPI-anchored protein may be required in the fusion process. To distinguish between these possibilities, we have begun to identify the GPI-anchored proteins on the egg surface. The authors have identified one egg GPI-anchored protein as CD55, an approximately 70 kDa complement regulatory protein. It has previously been found that CD55-knockout mice are fertile, demonstrating that CD55 is not essential for fertilization. This finding also means that the presence of the full complement of egg GPI-anchored proteins is not necessary for gamete fusion. Other egg GPI-anchored proteins acting in the fusion process can now be investigated, with the goal of understanding the mechanism of their function in sperm-egg fusion.

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created: 2003-04-16 07:53:49 by: Aaron J Hsueh, hsuehlab   email:
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last update: 2012-02-08 12:32:18 by: Aaron J Hsueh, hsuehlab   email:

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