SCHISTOSOMA-MANSONI - SM23 IS A TRANSMEMBRANE PROTEIN THAT ALSO CONTAINS A GLYCOSYLPHOSPHATIDYLINOSITOL ANCHOR

Sm23, a surface protein of the human parasite Schistosoma mansoni, bel ongs to the family of ''cysteine-rich, hydrophobic proteins,'' which a re expressed on mammalian hematopoietic cells or tumor cells. Sm23 sha res the highly conserved hydrophobicity profile of these proteins, whi ch predicts four transmembrane segments, but is in addition linked to the membrane by a glycosylphosphatidylinositol (GPI) anchor. Our resul ts suggest that Sm23 uses both the potential transmembrane domains and the GPI anchor for membrane insertion: (a) Sm23 was not released from the surface after cleavage with phosphatidylinositol-specific phospho lipase C (PIPLC). (b) In a Triton X-114 phase-separation system, nativ e [H-3]ethanolamine- or [S-35]methionine-labeled Sm23 partitioned into the detergent phase. Upon removal of the GPI anchor by PIPLC, the maj ority of the molecules stayed in the detergent-phase as expected of a transmembrane protein. (c) When full-length recombinant Sm23 was trans cribed and translated in vitro, the polypeptide chain was inserted int o microsomal membranes: Sm23 stayed associated with the membranes when they were incubated with carbonate buffer at pH 11.5, and membrane bo und Sm23 was protected from digestion with proteinase K. (d) Recombina nt Sm23, when expressed in the baculovirus expression system, was tran sported to the surface of infected insect cells, and similarly to the native protein it was not released from these cells after cleavage wit h PIPLC. (C) 1994 Academic Press, Inc.