Originally published In Press as doi:10.1074/mcp.M600035-MCP200 on April 7, 2006.
Molecular & Cellular Proteomics 5:1286-1299, 2006.
© 2006 by The American Society for Biochemistry and Molecular Biology, Inc.
Research
Identification and Stoichiometry of Glycosylphosphatidylinositol-anchored Membrane Proteins of the Human Malaria Parasite Plasmodium falciparum*,S
Paul R. Gilson , ,
Thomas Nebl , ,
Damjan Vukcevic ,¶,
Robert L. Moritz||,
Tobias Sargeant ,¶,
Terence P. Speed ,
Louis Schofield ,** and
Brendan S. Crabb ,**,
From The Walter and Eliza Hall Institute of Medical Research, Melbourne, Victoria 3050, Australia, ¶ Department of Medical Biology, The University of Melbourne, Parkville, Victoria 3010, Australia,, and || Joint Proteomics Laboratory, Ludwig Institute for Cancer Research and the Walter and Eliza Hall Institute of Medical Research, Melbourne 3052, Australia
Most proteins that coat the surface of the extracellular forms of the human malaria parasite Plasmodium falciparum are attached to the plasma membrane via glycosylphosphatidylinositol (GPI) anchors. These proteins are exposed to neutralizing antibodies, and several are advanced vaccine candidates. To identify the GPI-anchored proteome of P. falciparum we used a combination of proteomic and computational approaches. Focusing on the clinically relevant blood stage of the life cycle, proteomic analysis of proteins labeled with radioactive glucosamine identified GPI anchoring on 11 proteins (merozoite surface protein (MSP)-1, -2, -4, -5, -10, rhoptry-associated membrane antigen, apical sushi protein, Pf92, Pf38, Pf12, and Pf34). These proteins represent 94% of the GPI-anchored schizont/merozoite proteome and constitute by far the largest validated set of GPI-anchored proteins in this organism. Moreover MSP-1 and MSP-2 were present in similar copy number, and we estimated that together these proteins comprise approximately two-thirds of the total membrane-associated surface coat. This is the first time the stoichiometry of MSPs has been examined. We observed that available software performed poorly in predicting GPI anchoring on P. falciparum proteins where such modification had been validated by proteomics. Therefore, we developed a hidden Markov model (GPI-HMM) trained on P. falciparum sequences and used this to rank all proteins encoded in the completed P. falciparum genome according to their likelihood of being GPI-anchored. GPI-HMM predicted GPI modification on all validated proteins, on several known membrane proteins, and on a number of novel, presumably surface, proteins expressed in the blood, insect, and/or pre-erythrocytic stages of the life cycle. Together this work identified 11 and predicted a further 19 GPI-anchored proteins in P. falciparum.
 To whom correspondence should be addressed: The Walter and Eliza Hall Inst. of Medical Research, 1G Royal Parade, Parkville, Victoria 3050, Australia. Tel.: 61-3-9345-2555; Fax: 61-3-9347-0852; E-mail: crabb{at}wehi.edu.au

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Copyright © 2006 by the American Society for Biochemistry and Molecular Biology.
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