Endoplasmic Reticulum Envelope Research Articles

Targeting and membrane insertion into the endoplasmic reticulum membrane of Saccharomyces cerevisiae essential protein Rot1

Rot1 is an essential yeast protein that has been related to cell wall biosynthesis, actin cytoskeleton dynamics and protein folding. Rot1 is an N-glycosylated protein anchored to the nuclear envelope-endoplasmic reticulum (ER) membrane by a transmembrane domain at its C-terminal end. Rot1 is translocated to the ER by a post-translational mechanism. Here, we investigate the protein domain required to target and translocate Rot1 to the ER membrane. We found that several deletions of the N-terminal region of Rot1 prevented neither membrane targeting nor the insertion of this protein. Interestingly, we obtained the same results when different truncated forms in the C-terminal transmembrane domain were analyzed, suggesting the presence of an internal topogenic element that is capable of translocating Rot1 to the ER. To identify this sequence, we generated a combination of N- and C-terminal deletion mutants of Rot1 and we investigated their insertion into the membrane. The results show that two regions, amino acids 26-60 and 200-228, are involved in the post-translational translocation of Rot1 across the ER membrane.

Giardia lamblia: Traffic of a Trophozoite Variant Surface Protein and a Major Cyst Wall Epitope during Growth, Encystation, and Antigenic Switching

Both trophozoites and cysts of Giardia lamblia have unique outer surfaces that protect them from very different hostile environments. However, little is known about the transport of these important molecules to the cell surface. We used monospecific anti-recombinant TSA 417 antibodies and mAb 8C5 in double label immunoelectron microscopy to compare the localization and transport of this major trophozoite surface antigen (TSA) with that of a prominent cyst wall epitope during vegetative growth, encystation, and antigenic switching in vitro. TSA 417 is a marker of the constitutive transport of the major plasma membrane protein, while the 8C5 epitope traces a differentiation-regulated secretory pathway to the cyst wall. Both proteins localized to the nuclear envelope endoplasmic reticulum (ER) cisternae, ER, and cytoplasmic membrane cisternae, reflecting their site of synthesis, but only the differentiation-specific epitope 8C5 localized to the encystation-specific vesicles (ESV). These large secretory vesicles form only during encystation and transport cyst antigens to the nascent wall. In contrast, only TSA 417 was found on the outer surface of the plasmalemma of trophozoites and encysting cells and underlaying the walls of many cysts, while only 8C5 localized to the cyst wall. As encystation progressed, TSA 417 disappeared from the plasmalemma and increased in the lysosome-like PV and other large cytoplasmic vesicles. In contrast to their segregation in the ESV and on the cell surface, both TSA 417 and 8C5 were found in the peripheral vesicles, presumably an endocytic compartment. We propose that this may be the initiation of a stage in differentiation-driven antigenic switching of TSA 417, in which this antigen is no longer synthesized or exported to the plasmalemma, but is taken back inside the cell.

Nuagematerial and the origin of dense-core vesicles in oocytes ofNassarius reticulatus(Mollusca, Gastropoda)

Summary In late previtellogenic oocytes nuage material accumulates in the vicinity of the nucleus and is often seen to be intimately associated with cisternae of endoplasmic reticulum (ER). A similar association observed in Ilyanassa has given rise to the proposition that nuage granules become completely enclosed in an envelope of ER and that this structure is transformed into so-called double-membrane vesicles, organelles which have only been found in eggs of Ilyanassa and Nassarius. This study provides evidence that, in Nassarius, the association of nuage material with ER has a temporary character. At later stages the nuage granules dissociate from the ER, move away from the nucleus, and become surrounded by mitochondria. Eventually they disintegrate. Evidence is presented that double-membrane vesicles originate from cisternae of ER by the accumulation and transformation of material within the lumen of the cisternae. Since only a single membrane is present in these vesicles, and a dense core is consistently found if the appropriate fixation is employed, I suggest that these vesicles be called dense-core vesicles instead of double-membrane vesicles.

Ultrastructure of the zoospore of Blastocladia ramosa (Blastocladiales)

Species of Blastocladia, unlike other members of the order Blastocladiales, have an obligately fermentative metabolism. The ultrastructure of the zoospores of Blastocladia ramosa is described and compared with that of zoospores of aerobic members of the Blastocladiales. Zoospores of Blastocladia ramosa are structurally similar to other blastocladialean zoospores in that they have (i) a nuclear cap composed of aggregated ribosomes delimited by an envelope of endoplasmic reticulum; (ii) a posteriorly located nucleus; (iii) nine sets of microtubules that surround the nucleus and nuclear cap; and (iv) a striated rootlet associated with the kinetosome. Zoospores of Blastocladia ramosa differ by having mitochondria with unusual fine structure and by lacking microbodies and microbody – lipid globule complexes. As shown by enzymatic digestion and staining, glycogen appears to be the primary storage material in the zoospore. Structures which resemble water expulsion vacuoles are present. The morphological differences between zoospores of Blastocladia ramosa and other zoospores of the Blastocladiales appear to be related to physiological differences.

The ultrastructure of Helicodictyon planctonicum (Chlorophyceae)

Abstract Helicodictyon planctonicum (Whitford) Whitford & Schumacher (Chlorophyta) is a rare freshwater alga usually assigned to the order Chaetophorales but also reported to show features resembling members of the order Chlorococcales. In order to help clarify its phylogenetic position, an ultra-structural study was performed. The filamentous branching pattern and absence of a perinuclear envelope of endoplasmic reticulum during mitosis in H. planctonicum resemble features found among members of the Chaetophorales. However, the first and second cell divisions are perpendicular to each other as is common among chlorococcalean algae (Biebel, 1968). Additional chlorococcalean features exhibited by H. planctonicum include pyrenoids traversed by thylakoids, centrioles at the division plane following mitosis, and cytokinesis by furrowing. Therefore, it appears that H. planctonicum may be more closely allied with the Chlorococcales. However, failure to observe a phycoplast suggests Helicodictyon should be exami...