Leishmania mexicana mutants lacking glycosylphosphatidylinositol (GPI): Protein transamidase provide insights into the biosynthesis and functions of GPI-anchored proteins

The major surface proteins of the parasitic protozoon Leishmania mexicana a re anchored to the plasma membrane by glycosylphosphatidylinositol (GPI) an chors. We have cloned the L, mexicana GPI8 gene that encodes the catalytic component of the GPI:protein transamidase complex that adds GPI anchors to nascent cell surface proteins in the endoplasmic reticulum. Mutants lacking GPI8 (Delta GPI8) do not express detectable levels of GPI-anchored protein s and accumulate two putative protein-anchor precursors. However, the synth esis and cellular levels of other non-protein-linked GPIs, including lipoph osphoglycan and a major class of free GPIs, are not affected in the Delta G PI8 mutant. Significantly, the Delta GP18 mutant displays normal growth in liquid culture, is capable of differentiating into replicating amastigotes within macrophages in vitro, and is infective to mice. These data suggest t hat GPI-anchored surface proteins are not essential to L, mexicana for its entry into and survival within mammalian host cells in vitro or in vivo and provide further support for the notion that free GPIs are essential for pa rasite growth.