A surface antigen of Giardia lamblia with a glycosylphosphatidylinositol anchor.
Since Giardia lamblia trophozoites are exposed to high concentrations of fatty acids in their human small intestinal milieu, we determined the pattern of incorporation of [3H]palmitic acid and myristic acid into G. lamblia proteins. The pattern of fatty acylation was unusually simple since greater than 90% of the Giardia protein biosynthetically labeled with either [3H]palmitate or myristate migrated at approximately 49 kDa (GP49) in reducing sodium dodecyl sulfate-polyacrylamide gel electrophoresis during both growth and differentiation. GP49, which partitions into the Triton X-114 detergent phase, is localized on the cell surface since it is 125I-surface-labeled. GP49 was also biosynthetically labeled with [14C]ethanolamine and [3H]myoinositol, suggesting that it has a glycosylphosphatidylinositol (GPI) anchor. Moreover, phospholipase A2 (PLA2) or mild alkaline treatment released free fatty acids, indicating a diacylglycerol moiety with ester linkages. Finally, a 3H- and 14C-labeled species was released by nitrous acid deamination from [14C]palmitate- and [3H]myoinositol-labeled GP49. The GPI anchor of GP49 is unusual, however, because purified GP49 was cleaved by Bacillus cereus phosphatidylinositol (PI)-specific PLC, but not by Staphylococcus aureus PI-PLC, or plasma PLD, and did not react with antibody against the variant surface glycoprotein cross-reactive determinant. Moreover, the double-labeled deaminated GP49 anchor migrated faster than authentic PI in TLC and produced [3H]glycerophosphoinositol after deacylation. In contrast to the variable cysteine-rich G. lamblia surface antigens described previously, GP49 was identified in Western blots of every isolate tested, as well as in subclones of a single isolate which differ in expression of a major cysteine-rich 85/66-kDa surface antigen, which does not appear to be GPI-anchored. These observations suggest that GP49, the first common surface antigen to be described in G. lamblia, may play an important role in the interaction of this parasite with its environment.
Das
S
S
Department of Pathology, University of California, San Diego 92103.
Traynor-Kaplan
A
A
Reiner
D S
DS
Meng
T C
TC
Gillin
F D
FD
D000215:Acylation; D000818:Animals; D000954:Antigens, Surface / Q000737:chemistry / Q000378:metabolism*; D016829:Giardia lamblia / Q000254:growth & development / Q000276:immunology*; D006017:Glycolipids / Q000378:metabolism*; D017261:Glycosylphosphatidylinositols; D006863:Hydrogen-Ion Concentration; D008562:Membrane Glycoproteins / Q000737:chemistry / Q000276:immunology* / Q000378:metabolism; D008744:Methylamines / Q000737:chemistry; D009608:Nitrous Acid / Q000737:chemistry; D043265:Phosphatidylinositol Diacylglycerol-Lyase; D010716:Phosphatidylinositols / Q000378:metabolism*; D054801:Phosphoinositide Phospholipase C; D010741:Phospholipases A / Q000494:pharmacology; D054467:Phospholipases A2; D010727:Phosphoric Diester Hydrolases / Q000378:metabolism; D011499:Protein Processing, Post-Translational; D015800:Protozoan Proteins / Q000737:chemistry / Q000276:immunology / Q000378:metabolism*
D000954:Antigens, Surface; D006017:Glycolipids; D017261:Glycosylphosphatidylinositols; D008562:Membrane Glycoproteins; D008744:Methylamines; D010716:Phosphatidylinositols; D015800:Protozoan Proteins; C027451:methylamine; D010741:Phospholipases A; D054467:Phospholipases A2; D010727:Phosphoric Diester Hydrolases; D054801:Phosphoinositide Phospholipase C; D043265:Phosphatidylinositol Diacylglycerol-Lyase; D009608:Nitrous Acid