4. Endoplasmic reticulum

Although electron microscopy revealed similar structures to endoplasmic reticulum (ER), there is still a controversy concerning the real presence of this organelle in Giardia. The cloning and characterization of SRα, a receptor for signal recognition particle (SRP) as well as the use of binding immunoglobulin protein (BiP), a ER resident 70 kDa heat shock protein, allowed the identification of an extensive membrane system in G. intestinalis [23]. So far, the presence of an ER has been demonstrated in the parasite; moreover, three genes for protein disulfide isomerase (PDI) in Giardia were cloned and characterized, and its products were localized into ER [24]. Electron microscopy cytochemistry for glucose-6-phosphate, a resident ER enzyme, allowed the observation of an extensive reticular system in this parasite [17]. The ER presents a complex, bilaterally symmetrical organization that is distributed from the nuclear envelopes throughout the cell body (Figure 3). The presence of this archetypical eukaryotic organelle in Giardia has been called into question [16]. The ER in giardia trophozoites is composed by little tinny cisternae (Figure 3c), while during the encystation process, there is an increase in the ER cisternae number [17].

Although G. intestinalis has a conventional ER concerning the secretory trafficking, some elements are missing entirely, such as the post-translational modification machinery. The calnexin-calreticulin machinery, which acts in the quality control of N-glycosylated secreted proteins, is absent [25]. An extensive genomic and biochemical analyses demonstrated that the parasite lacks several nucleotide sugar transporters [26]. Thus, Asn-linked glycosylation in the giardial ER is limited to the addition of GlcNAc1–2 to proteins. A coordinated work among conserved machinery for translocation [27], and chaperones and members of the PDI family [28] support the co-translational import and folding of secreted proteins. Giardial PDIs play a major role in assisting the folding of the cyst wall proteins (CWPs) [29].

It was proposed that the Giardia's reticulum would be a tubule-vesicular network with ER functions as well as endosomes and lysosomes activities by connections with the peripheral vesicles (Figure 4) [19]. Based on that, the ER of G. intestinalis possesses different functions (e.g., protein synthesis, endocytic activities and extracellular material degradation), since it is a pluripotent compartment [19].
