A Contiguous Compartment Functions as Endoplasmic Reticulum and Endosome/Lysosome in Giardia lamblia

Abstract

The dynamic evolution of organelle compartmentalization in eukaryotes and how strictly compartmentalization is maintained are matters of ongoing debate. While the endoplasmic reticulum (ER) is classically envisioned as the site of protein cotranslational translocation, it has recently been proposed to have pluripotent functions. Using transfected reporter constructs, organelle-specific markers, and functional enzyme assays, we now show that in an early-diverging protozoan, Giardia lamblia, endocytosis and subsequent degradation of exogenous proteins occur in the ER or in an adjacent and communicating compartment. The Giardia endomembrane system is simple compared to those of typical eukaryotes. It lacks peroxisomes, a classical Golgi apparatus, and canonical lysosomes. Giardia orthologues of mammalian lysosomal proteases function within an ER-like tubulovesicular compartment, which itself can dynamically communicate with clathrin-containing vacuoles at the periphery of the cell to receive endocytosed proteins. These primitive characteristics support Giardia's proposed early branching and could serve as a model to study the compartmentalization of endocytic and lysosomal functions into organelles distinct from the ER. This system also may have functional similarity to the retrograde transport of toxins and major histocompatibility complex class I function in the ER of mammals.

The Unique Endosomal/Lysosomal System of Giardia lamblia

“…Possibly supporting this hypothesis, giardial clathrin and AP-1 were observed not only on ER-exist sites but also in PVs (Marti et al, 2003a;Marti et al, 2003b;Touz et al, 2004;Gaechter et al, 2008). As we said, the ER tubular-vesicular network apparently extends to and contacts the PVs in the periphery of the cell (Abodeely et al, 2009b) but it was recently reported that no ER membranes invade the space occupied by PVs (Faso and Hehl, 2011). An explanation that reconciles these observations might be that at least some of the clathrin-dependent trafficking in Giardia involves tubular carriers that extend from the ER-exit sites to the peripheral cytoplasm until they meet with distally located PVs.…”

“…Recently, it was shown that a mechanism of receptor-mediated endocytosis occurs in this organism, with specific molecules selectively directed to the PVs through a classical endocytic mechanism (Rivero et al, 2010;. These data suggest that uptake of soluble material into PVs is not selective at this step but is still capable of redirecting specific molecules to the TVN (Hernandez et al, 2007;Abodeely et al, 2009b). In terms of receptor-mediated endocytosis, movement between vesicles could be observed (Figure 4), which suggests, not only that Giardia possesses a refined and conserved mechanism of endocytosis, but also that the PVs population might not be as homogeneous as was thought but rather organized depending on their functions (Rivero & Touz, unpublished).…”

“…It was observed that, after internalization, some fluid-phase markers translocated rapidly to the ER or to an associated membrane compartment termed the tubulo-vesicular network (TVN) (Abodeely et al, 2009). Moreover, the presence of protease functions within the TVN, plus 3-D reconstruction and electron microscopy tomography of trophozoites stained for acid phosphatase and glucose-6-phosphatase, suggest that there might be a connection between some vesicles and profiles of the ER (Lanfredi-Rangel et al, 1998;Abodeely et al, 2009a). However, a recent work showed, by immunofluorescence and 3-D reconstruction, that the ER membranes are found throughout the cytoplasm, but do not permeate the space occupied by PVs (Faso and Hehl, 2011).…”

“…A family of three cysteine protease genes (CP1, CP2, and CP3) has been shown to encode members of the cathepsin B subgroup of the peptidase family C1 (Ward et al, 1997), and soluble CP2 has been found in PVs and ER of trophozoites (Ward et al, 1997;Abodeely et al, 2009a). Also, AcPh activity has been examined cytochemically, revealing communication of the PVs with the ER (Feely and Dyer, 1987;Lanfredi-Rangel et al, 1998;de Souza et al, 2004).…”

“…Similar to the SNAREs, there is a remarkable reduction in the members of this family, with only seven Rab proteins being present in the Giardia genome (Morrison et al, 2007). The giardial Rab11 has been localized to the PVs and cytoplasm but relocalizes to the ESVs during encystation (Morrison et al, 2007;Abodeely et al, 2009b). A Giardia Rab1 that localizes to ER-exit sites and PVs is also associated with the ESVs during encystation (Langford et al, 2002).…”