CELL BIOLOGY OF THE PRIMITIVE EUKARYOTE GIARDIA-LAMBLIA

Giardia lamblia is an extremely primitive or early-diverging eukaryote that has been considered to have no typical ER or Golgi apparatus, al though it is a complex and highly developed cell. Both the trophozoite and cyst have unusual surface proteins that enable these stages to su rvive in very different and hostile environments. We found that G. lam blia forms novel encystation-specific secretory vesicles and can sort cyst wall proteins to a regulated secretory pathway distinct from the constitutive pathway used to transport the variable cysteine-rich prot ein to the trophozoite surface. Our studies, utilizing novel ultrastru ctural methods that preserve the endomembranes, as well as IEM, suppor t the idea that G. lamblia has many of the endomembrane protein transp ort elements and sorting functions of higher cells and that these appe ared very early in the evolution of eukaryotic cells.