Biosynthesis of glycophospholipid anchors
Abstract (Summary)Kinetic data, and data from yeast sec mutants indicate that anchor addition occurs in the rough endoplasmic reticulum (RER). This raises major topological questions, since most of the anchor precursors are present in the cytosol. Only rare cloned transfectants expressed GPL-anchored AP on their surface, and exactly those clones were found to synthesize DPM. Fusion of B4-2-1 with Thy-1- class E mutant thymoma deficient in DPM synthesis produced stable heterokaryons which expressed lipid-anchored Thy-1 and synthesized DPM. These data show that DPM is an essential donor of mannose residue(s) for GPL anchor biosynthesis, and suggest that two gene products are required for DPM synthesis. In order to learn more about biosynthesis of the GPL anchor, mutant mouse L-cells (LM-TK-) were studied. Unlike L929 cells, LM-TK- cells synthesize, but do not add glycophospholipid anchors to GPL-anchored proteins. ^3H-mannose-labeled glycolipids for L929, LM-TK-, S49 wild-type and S49 class H lymphoma mutants showed striking similarities between the two wild-type cells by contrast to the mutants. The differences pertained to putative anchor precursor glycolipids which were present in the wild-type cells, but absent in the two mutants. These and related studies th erefore show that: the GPL anchor unit acquires one or more mannose residues from DPM in the lumen of the RER; a functional GPL-anchor is not added to AP in the absence of these mannose residue(s); the biochemical defects in class E and B4-2-1 cells are distinct; some factor(s) required from DPM synthesis in B4-2-1 x class E heterokaryons are functional only in vivo. Additionally, LM-TK- cells belong to lymphoma complementation group H by reference to Thy-1- T lymphoma mutants; when caused to synthesize AP by transfection, these cells produce an anchorless, hydrophilic species; AP synthesized by these cells is rapidly degraded in lysosomes; the anchor synthesis defect in these cells appears to reflect the absence of a putative anchor precursor lipid which is also absent in class H cells. (Abstract shortened with permission of author.
School Location:USA - Ohio
Source Type:Master's Thesis
Date of Publication:01/01/1990