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Volume 272, Number 46, Issue of November 14, 1997 pp. 29212-29221
©1997 by The American Society for Biochemistry and Molecular Biology, Inc.

Characterization of a Novel, Stage-specific, Invariant Surface Protein in Trypanosoma brucei Containing an Internal, Serine-rich, Repetitive Motif

(Received for publication, August 6, 1997)

Derek P. Nolan , David G. Jackson ^§ , Henry J. Windle ^¶ , Annette Pays , Maurice Geuskens , Alain Michel , H. Paul Voorheis ^¶ and Etienne Pays

From the Department of Molecular Biology, University of Brussels, 67 rue des Chevaux, B-1640 Rhode St. Genèse, Belgium, the ^§ Institute of Molecular Medicine, John Radcliffe Hospital, Oxford OX3 9DU, UK, the ^¶ Department of Biochemistry, Trinity College, Dublin 2, Ireland, and the  Laboratoire de Chimie Biologique, Universite de Mons-Hainaut, B-7000 Mons, Belgium

A new surface membrane protein, invariant surface glycoprotein termed ISG₁₀₀, was identified in Trypanosoma brucei, using catalyzed surface, radioiodination of intact cells. This integral membrane glycoprotein was purified by a combination of detergent extraction, lectin-affinity, and ion-exchange chromatography followed by preparative SDS-polyacrylamide gel electrophoresis. The protein was expressed only in bloodstream forms of the parasite, was heavily N-glycosylated, and was present in different clonal variants of the same serodeme as well as in different serodemes. The gene for this protein was isolated by screening a cDNA expression library with antibodies against the purified protein followed by screening of a genomic library. The nucleotide sequence of the gene (4050 base pairs) predicted a highly reiterative polypeptide containing three distinct domains, a unique N-terminal domain of about 10 kDa containing three potential N-glycosylation sites, which was followed by a large internal domain consisting entirely of 72 consecutive copies of a serine-rich, 17-amino acid motif (~113 kDa) and terminated with an apparent transmembrane spanning region of about 3.3 kDa. The internal repeat region of this gene (3672 base pairs) represents the largest reiterative coding sequence to be fully characterized in any species of trypanosome. There was no significant homology with other known proteins, and overall the predicted protein was extremely hydrophobic. Unlike the genes for other surface proteins, the gene encoding ISG₁₀₀ was present as a single copy. Although present in the flagellar pocket, ISG₁₀₀ was predominantly associated with components of the pathways for endo/exocytosis, such as intracellular vesicles located in the proximity of the pocket as well a large, electron-lucent perinuclear digestive vacuole.

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