Molecular cloning and characterization of a novel repeat-containing Leishmania major gene, ppg1, that encodes a membrane-associated form of proteophosphoglycan with a putative glycosylphosphatidylinositol anchor

Leishmania parasites secrete a variety of proteins that are modified by pho sphoglycan chains structurally similar to those of the cell surface glycoli pid lipophosphoglycan. These proteins are collectively called proteophospho glycans. We report here the cloning and sequencing of a novel Leishmania ma jor proteophosphoglycan gene, ppg1. It encodes a large polypeptide of appro ximately 2300 amino acids. The N-terminal domain of approximately 70 kDa ex hibits 11 imperfect amino acid repeats that show some homology to promastig ote surface glycoproteins of the psa2/gp46 complex. The large central domai n apparently consists exclusively of approximately 100 repetitive peptides of the sequence APSASSSSA(P/S)SSSSS(+/-S). Gene fusion experiments demonstr ate that these peptide repeats are the targets of phosphoglycosylation in L eishmania and that they form extended filamentous structures reminiscent of mammalian mucins. The C-terminal domain contains a functional glycosylphos phatidylinositol anchor addition signal sequence, which confers cell surfac e localization to a normally secreted Leishmania acid phosphatase when fuse d to its C terminus. Antibody binding studies show that the ppg1 gene produ ct is phosphoglycosylated by phosphoglycan repeats and cap oligosaccharides . In contrast to previously characterized proteophosphoglycans, the ppg1 ge ne product is predominantly membrane-associated and it is expressed on the promastigote cell surface. Therefore this membrane-bound proteophosphoglyca n may be important for direct host-parasite interactions.