Secretory protein trafficking in Giardia intestinalis

Abstract

SummaryEarly diverged extant organisms, which may serve as convenient laboratory models to look for and study evolutionary ancient features of eukaryotic cell biology, are rare. The diplomonad Giardia intestinalis , a protozoan parasite known to cause diarrhoeal disease, has become an increasingly popular object of basic research in cell biology, not least because of a genome sequencing project nearing completion. Commensurate with its phylogenetic status, the Giardia trophozoite has a very basic secretory system and even lacks hallmark structures such as a morphologically identifiable Golgi apparatus. The cell's capacity for protein sorting is nevertheless unimpeded, exemplified by its ability to cope with massive amounts of newly synthesized cyst wall proteins and glycans, which are sorted to dedicated Golgi-like compartments termed encystation-specific vesicles (ESVs) generated from endoplasmic reticulum (ER)-derived transport intermediates. This soluble bulk cargo is kept strictly separate from constitutively transported variant surface proteins during export, a function that is dependent on the stage-specific recognition of trafficking signals. Encysting Giardia therefore provide a unique system for the study of unconventional, Golgi-independent protein trafficking mechanisms in the broader context of eukaryotic endomembrane organization and evolution.

Characterization of SNAREs Determines the Absence of a Typical Golgi Apparatus in the Ancient Eukaryote Giardia lamblia

“…G. lamblia trophozoites possess an interesting secretory system in which a morphologically identifiable Golgi apparatus seems to be absent, although the packaging and sorting functions of this organelle are obvious in this organism (4,5). For example, transport to the plasma membrane (PM) 2 and release into the culture medium of variant-specific surface proteins (VSPs) (6,7), as well as trafficking of both membrane and soluble enzymes to peripheral vacuoles (PVs), which are thought to perform both lysosomal and endosomal activities (4,8), are evidence for constitutive protein transport.…”

“…During the last decade, many proteins involved in Giardia secretory pathways have been identified and characterized: endoplasmic reticulum (ER) chaperones such as BiP/GRP78 (14) and protein-disulfide isomerase (15), cyst wall proteins 1-3 (CWPs) (11,12,16), encystation-specific cysteine protease (17), granule-specific protein (17), and giardins (18), among others (5). Recently, the completion of the Giardia genome project (1) has allowed the identification of a number of molecules involved in protein trafficking (i.e.…”

Encystation commitment inGiardia duodenalis: a long and winding road

“…The release of ESV contents and CWF formation/assembly may occur stepwise in late encysting cells, i.e., CWP and HCNCp (glyco)polypeptide exposure on the cell surface precedes their co-polymerization with the [GalNAc(β1➝3)GalNAc(β1➝3)] n complex (Argüello-García et al, 2002). This proposal is supported by independent estimations of the time for ESV content release (up to 1 min) (Hehl & Marti, 2004) compared with that required for complete CW assembly (5-6 h.), a process occurring by fibril tip growth (Erlandsen et al, 1996;Argüello-García et al, 2002) and by the presence of CW (glyco) polypeptides in 15-to-100 nm-sized protrusions laid on the surface of encysting cells (Erlandsen et al, 1996) which do not display typical CWF architecture. The cytolocalization of cyst wall synthase (CWS) activity in encysting cells will provide further insights into the process of CW formation.…”

“…Several studies have aimed at defining the transport pathway of CW components to the cell surface. Giardia lacks typical Golgi dictyosomes but possesses a basic framework of the vesicular transport system that includes coatomers (COP) I and II (Golgi-and ER-specific, respectively), clathrins, two adaptor protein (AP) complexes and key factors for vesicular targeting and membrane fusion processes such as members of the soluble N-ethylmaleimide-sensitive factor attachment protein receptor (SNARE) and Rab families (Luján & Touz, 2003;Hehl & Marti, 2004). Encystation-specific vesicles (ESV) are specialized secretory granules that mature during their traffic from ER to the cell surface that act as the equivalents of late Golgi structures and are also a hallmark of encystation induction.…”

“…CWP2 processing would be also performed by another ESV-contained cathepsin B-like cysteine protease (orf EAA 37074; DuBois et al, 2008) but not so for the peripheral vesicle (PV)-contained cathepsin C-like encystation-specific cysteine protease (ESCP, orf EAA 36907;Touz et al, 2002). Near to the plasma membrane, ESV appears to carry out two processes: a) fusion with PV (Luján & Touz, 2003) that acidifies the cargo to limit CP2 activity and/or allow ESCP activity and favor other undefined processing mechanisms preceding the release of CW material on the cell surface; or b) fission (dispersal) into small secretory vesicles that eventually fuse with the plasma membrane to discharge its content (Hehl & Marti, 2004). Proposal 'a' is supported by the PV-to-ESV redistribution of CLH (Marti et al, 2003) while 'b' is consistent with microscope observations of the initial secretion of antigenic [(glyco)polypeptide] CW material as small protrusions on the surface of encysting cells (Erlandsen et al, 1990(Erlandsen et al, , 1996.…”