The parasitic protozoan Giardia lamblia undergoes important changes to survive outside the
intestine of its host by differentiating into infective cysts. During encystation, three cyst wall
proteins (CWPs) are specifically expressed and concentrated within encystation-specific
secretory vesicles (ESVs). ESVs are electron-dense secretory granules that transport CWPs
before exocytosis and extracellular polymerization into a rigid cyst wall. Because secretory
granules form at the trans-Golgi in higher eukaryotes and because Giardia lacks an …