Secretory granule biogenesis and the organization of membrane compartments via SNARE proteins in Giardia lamblia.

Giardia lamblia trophozoites lack organelles typical of higher eukaryotes such as mitochondria, peroxisomes and compartments involved in intracellular protein trafficking and secretion, the Golgi apparatus and secretory granules. Nevertheless, the minimal machinery for protein transport and sorting is present in this parasite. When Giardia undergoes encystation, the biogenesis of secretory organelles necessary to transport cyst wall constituents to the cell surface takes place. Although both constitutive and regulated pathways for protein secretion exist in Giardia, little is known about the molecules and mechanisms specifically involved in vesicular docking and fusion. In higher eukaryotes, soluble N-ethylmaleimide-sensitive factor attachment protein receptor (SNARE) of the vesicle-associated membrane protein (VAMP) and syntaxin proteins play an essential role in these processes through the formation of complexes between proteins present on donor and target membranes. Recent studies in both vegetative and encysting trophozoites have provided interesting information regarding the secretory pathway of this important pathogen.