GIARDIA-LAMBLIA - ULTRASTRUCTURAL BASIS OF PROTEIN-TRANSPORT DURING GROWTH AND ENCYSTATION

Giardia, an early diverging eukaryote, is reported to have no Golgi ap paratus. Moreover, the structural basis for its ability to sort key pr oteins and transport them to the trophozoite plasma membrane or to the extracellular wall during encystation is not well-understood. Therefo re, we have used ultrastructural techniques that enhance the endomembr ane system to evaluate the presence and relationships of cytoplasmic o rganelles and structures that correspond to those present in higher ce lls. In addition to the perinuclear cisternae, we found rough endoplas mic 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 compartm ents. Importantly, many of these membrane elements appeared to be capt ured in the process of budding (or fusing). These elements of the endo membrane system are present during both vegetative growth and encystat ion of Giardia lamblia. In contrast, the encystation-specific vesicles (ESV) are novel large regulated secretory vesicles that transport cys t antigens to the nascent wall. The present studies suggest that ESV m ay have unusual pathways of formation and traffic. Our findings suppor t the idea that Giardia, a primitive parasite, has complex structures for protein transport. The elements that show similarities to higher c ells may have evolved early, while those that differ may represent bio logic fossils or specializations for the parasitic life cycle. (C) 199 4 Academic Press, Inc.