MECHANISMS OF PROTEIN SORTING AND TRAFFICKING IN GIARDIA LAMBLIA

TOUZ MC; MIRAS S; FELIZIANI C; ZAMPONI N; ROPOLO AS

Congreso; IX Congreso Argentino de Protozoología y Enfermedades Parasitarias.; 2011

Recent advances have been exploited to address fundamental questions related to secretory trafficking in protozoa parasites. Targeted gene disruptions and fusion protein expression have provided unexpected insights into both the function and assembly of the essential parasite secretory pathway. The giardial secretory pathway represents the intracellular route for proteins involved in surface-coat and immunity, lysosomal-specific trafficking and, during encystation, secretory granule formation required for cyst wall formation. Although Giardia trophozoite is able to distinguish between these three different pathways, it lacks the same secretory compartments as other eukaryotic cells challenging canonical models of secretory pathway organization. In this work, we showed evidence for a distributed secretory pathway in Giardia, suggested mechanisms that may regulate secretory compartment distribution, and discussed the implications of this unique secretory pathway for Giardia morphogenesis during differentiation. Giardia lacks a distinctive Golgi apparatus but it possesses a perinuclear area where the functions of the ER and Golgi co-localize spatially and temporally. It was reported that during encystation, Giardia trophozoites undergo the induction of Golgi enzyme activities that correlates with the appearance of a morphologically identifiable Golgi complex [1]. This induction of Golgi structure and carbohydrate-processing enzymatic activities (Golgi function) associate with the appearance of encystation-specific vesicles, which transport cyst wall components to the plasma membrane of the encysting cell and release their content to the cell surface during cyst wall formation. In order to localize the putative Golgi organelle with sorting function in vegetative and encysting trophozoites, we used the Golgi marker for mammalian cells NBD C6-Ceramide (Figure 1A-C). Further studies using Brefeldin A, a fungal metabolite that blocks protein secretion and perturbs the structure of the Golgi apparatus, showed that these perinuclear structures were reallocated to the ER suggesting that even in the absence of a morphological identifiable Golgi structure the Golgi functions are active in this parasite. Similarly, the functions of early/late endosomes and lysosomes appear to be fulfilled by the peripheral vacuoles (PVs). PVs have been shown to be acidic, contain hydrolase activities and are involved in endocytosis and protein degradation. Recently, we found that delivering of soluble and membrane hydrolases to the PVs depended on adaptor proteins and clathrin, suggesting that Giardia possesses molecular mechanisms for lysosomal protein trafficking similar to those of other eukaryotes [2]. In almost all eukaryotes, once the proteins arrive to the last Golgi compartment, they are recruiting in vesicles to be transported to a specific organelle, delivered to the plasma membrane or secreted. Variant-specific surface proteins (VSPs) are a family of related proteins that coat the entire surface of Giardia trophozoites including the flagella. So far, all VSPs are membrane proteins that contain similar transmembrane amino acid sequences and a very conserved terminate CRGKA cytoplasmic domain. We recently found that posttranslational modifications of the CRGKA affected VSP function but not surface localization [3, 4]. Remain to be elucidating whether in the absence of sorting signal the default pathway transports proteins to the plasma membrane or is the transmembrane domain that is implicated in VSPs