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

Abstract

Giardia is a eukaryotic protozoal parasite with unusual characteristics, such as the absence of a morphologically evident Golgi apparatus. Although both constitutive and regulated pathways for protein secretion are evident in Giardia, little is known about the mechanisms involved in vesicular docking and fusion. In higher eukaryotes, soluble N-ethylmaleimide-sensitive factor attachment protein receptors (SNAREs) of the vesicle-associated membrane protein and syntaxin families play essential roles in these processes. In this work we identified and characterized genes for 17 SNAREs in Giardia to define the minimal set of subcellular organelles present during growth and encystation, in particular the presence or not of a Golgi apparatus. Expression and localization of all Giardia SNAREs demonstrate their presence in distinct subcellular compartments, which may represent the extent of the endomembrane system in eukaryotes. Remarkably, Giardia SNAREs, homologous to Golgi SNAREs from other organisms, do not allow the detection of a typical Golgi apparatus in either proliferating or differentiating trophozoites. However, some features of the Golgi, such as the packaging and sorting function, seem to be performed by the endoplasmic reticulum and/or the nuclear envelope. Moreover, depletion of individual genes demonstrated that several SNAREs are essential for viability, whereas others are dispensable. Thus, Giardia requires a smaller number of SNAREs compared with other eukaryotes to accomplish all of the vesicle trafficking events that are critical for the growth and differentiation of this important human pathogen.

Neogenesis and maturation of transient Golgi-like cisternae in a simple eukaryote

“…Consistent with this, most of the machinery for biosynthesis of core N-glycans and all associated quality control modules for endoplasmic reticulum (ER) export are absent Samuelson et al, 2005). Despite the completely different organization of the trafficking system, a minimal set of universally conserved molecules has been identified (Dacks et al, 2003;Elias et al, 2008;Marti et al, 2003b), suggesting that the basic principles of membrane transport are conserved. During stage-differentiation to infectious cysts, trophozoites synthesize large amounts of cyst wall material (CWM), which is secreted to the surface of the cells in a regulated fashion to form a protective extracellular matrix.…”

The Unique Endosomal/Lysosomal System of Giardia lamblia

“…SNAREs (soluble N-ethylmaleimide-sensitive factor-attachment protein receptors) function in the final event of docking of vesicles with the target membrane, catalyzing the fusion of the opposing membranes. Compared to other organisms, Giardia has a relatively small number of SNARE proteins (Morrison et al, 2007;Elias et al, 2008). Seventeen putative SNAREs have been identified and partially characterized, with five representing QaSNAREs, five Qb-SNAREs, four Qc-SNAREs and three R-SNAREs.…”

“…Seventeen putative SNAREs have been identified and partially characterized, with five representing QaSNAREs, five Qb-SNAREs, four Qc-SNAREs and three R-SNAREs. Although some of these SNAREs localize to the PV area, their function has not been investigated and has rather been inferred from the participation of their orthologs in other cells (Elias et al, 2008). For example, different gSNAREs are present in the PVs/plasma membrane area, Qa1, gQa3, gQa5, gQb2, gQb4, gQb5, and gR3, with the presence of three different gQa SNAREs suggested to be involved in distinct pathways such as exocytosis, endocytosis, and PV-PV fusion (Elias et al, 2008).…”

Lysosomal protein trafficking in Giardia lamblia common and distinct features

“…Although some of these SNAREs localize to the PV area, their function has not been investigated and has rather been inferred from the participation of their orthologs in other cells (82). For example, different gSNAREs are present in the PVs/plasma membrane area, Qa1, gQa3, gQa5, gQb2, gQb4, gQb5, and gR3, with the presence of three different gQa SNAREs suggested to be involved in distinct pathways such as exocytosis, endocytosis, and PV-PV fusion (82).…”

“…Compared to other organisms, Giardia has a relatively small number of Rab and SNARE proteins (28,82). The giardial Rab11 has been localized to the PVs and cytoplasm but relocalizes to the ESVs during encystation (28,36).…”