Giardia lamblia: ultrastructural basis of protein transport during growth and encystation

Giardia, an early diverging eukaryote, is reported to have no Golgi apparatus. Moreover, the structural basis for its ability to sort key proteins and transport them to the trophozoite plasma membrane or to the extracellular wall during encystation is not well-understood. Therefore, we have used ultrastructural techniques that enhance the endomembrane system to evaluate the presence and relationships of cytoplasmic organelles and structures that correspond to those present in higher cells. In addition to the perinuclear cisternae, we found rough endoplasmic reticulum (ER), transitional elements, putative tubular-vesicular elements, Golgi-like smooth perinuclear membrane stacks, and lysosome-like peripheral vesicles. Moreover, we observed many small (50-80 nm) vesicles, many of which were coated, that resemble the small transport vesicles that carry proteins between successive ER and Golgi compartments. Importantly, many of these membrane elements appeared to be captured in the process of budding (or fusing). These elements of the endomembrane system are present during both vegetative growth and encystation of Giardia lamblia. In contrast, the encystation-specific vesicles (ESV) are novel large regulated secretory vesicles that transport cyst antigens to the nascent wall. The present studies suggest that ESV may have unusual pathways of formation and traffic. Our findings support the idea that Giardia, a primitive parasite, has complex structures for protein transport. The elements that show similarities to higher cells may have evolved early, while those that differ may represent biologic fossils or specializations for the parasitic life cycle.