Active and passive mechanisms drive secretory granule biogenesis during differentiation of the intestinal parasite Giardia lamblia.

NATALIA GOTTIG; ELIANA V ELÍAS; RODRIGO QUIROGA; MARÍA J NORES; ALBERTO J SOLARI; MARÍA C TOUZ; HUGO D LUJÁN

JOURNAL OF BIOLOGICAL CHEMISTRY

AMER SOC BIOCHEMISTRY MOLECULAR BIOLOGY INC

Año: 2006 vol. 281 p. 18156 - 18166

0021-9258

The parasitic protozoan Giardia lamblia undergoes importantchanges to survive outside the intestine of its host by differentiatinginto infective cysts. During encystation, three cyst wall proteins(CWPs) are specifically expressed and concentrated within encystation-specific secretory vesicles (ESVs). ESVs are electron-densesecretory granules that transport CWPs before exocytosis andextracellular polymerization into a rigid cyst wall. Because secretorygranules form at the trans-Golgi in higher eukaryotes andbecause Giardia lacks an identifiable Golgi apparatus, the aim ofthis work was to investigate the molecular basis of secretory granuleformation in Giardia by examining the role ofCWPsin this process.Although CWP1, CWP2, andCWP3are structurally similar in their26-kDa leucine-rich overlapping region, CWP2 is distinguished bythe presence of a 13-kDa C-terminal basic extension. In non-encystingtrophozoites, expression of different CWP chimeras showedthat the CWP2 basic extension is necessary for biogenesis of ESVs,which occurs in a compartment derived from the endoplasmic reticulum.Nevertheless, the CWP2 basic extension per se is insufficientto trigger ESV formation, indicating that other domains in CWPsare also required. We found that CWP2 is a key regulator of ESVformation by acting as an aggregation factor for CWP1 and CWP3through interactions mediated by its conserved region. CWP2 alsoacts as a ligand for sorting via its C-terminal basic extension. Thesefindings show that granule biogenesis requires complex interactionsamong granule components and membrane receptors.