Protein trafficking in Giardia lamblia

Abstract

SummaryGiardia , a protozoan parasite of humans and other vertebrates, is a common cause of intestinal disease worldwide. Besides its medical importance, Giardia is considered an excellent system to study the evolution of fundamental cellular processes because it belongs to the earliest branches of the eukaryotic lineage of descent. Giardia trophozoites lack organelles typical of higher eukaryotes such mitochondria, peroxisomes and compartments involved in intracellular protein trafficking and secretion, such as 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. Recent studies in both vegetative and encysting trophozoites have provided interesting information regarding the secretory pathway of this important human pathogen.

Encystation commitment inGiardia duodenalis: a long and winding road

“…Green fluorescent protein (GFP)-CWP1 chimeras have shown that the N-terminal domain may be needed for sorting CWP to secretory compartments and the LRR tandem repeats may help CWP to associate with CWF material (Hehl et al, 2000). The cysteinerich domain may aid CWP in forming disulfide-bonded heterodimers (Luján et al, 1995;Sun et al, 2003) that are concentrated in the ER lumen potentially promoting ESV biogenesis (Luján & Touz, 2003). This is supported by the fact that dithiothreitol reduces CWP complexes to monomers, inhibits ESV formation and reversibly converts these secretory granules into ERlike flattened cisternae (Reiner et al, 2001).…”

“…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.…”

“…CWP complexes are concentrated, stabilized, sorted and bud in ESV together with chaperons that allow their correct folding (by immunoglobulin heavy chain-binding protein (BiP)) and oligomerization (by protein disulfide isomerases (PDI) 1-3). Another ESV cargo protein (gGSP) helps to maintain low intra-ESV calcium levels to prevent premature assembly and to regulate exocytosis of CWP complexes (Luján & Touz, 2003). The release of CW material from ESV is carried out by exocytosis and appears to involve incomplete fusion between plasma and ESV membranes (Benchimol, 2004).…”

“…The trophozoite-to-cyst conversion process (encystation) of this parasite represents a primitive response to cellular stress. Likewise Giardia is a relatively simple eukaryote with reduced/cryptic structures such as mitosomes, Golgi-like apparatus and nucleolus (Tovar et al, 2003;Luján & Touz, 2003;Jiménez-García et al, 2008) coupled to a compact genome encoding a limited proteomic repertoire (Morrison et al, 2007). Nevertheless Giardia is considered as the most highly evolved and adaptedto-parasitism diplomonad on the basis of morphogenetic criteria (Keeling & Brugerolle, 2006).…”