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, although it is a complex and highly developed cell. Both the trophozoite and cyst have unusual surface proteins that enable these stages to survive in very different and hostile environments. We found that G. lamblia 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 protein to the trophozoite surface. Our studies, utilizing novel ultrastructural methods that preserve the endomembranes, as well as IEM, support the idea that G. lamblia has many of the endomembrane protein transport elements and sorting functions of higher cells and that these appeared very early in the evolution of eukaryotic cells.