Intracellular transport of a variant surface glycoprotein in Trypanosoma brucei

Reconstitution of Golgi Biogenesis in Permeabilized Trypanosoma brucei Cells

The protozoan parasite, Trypanosoma brucei, offers a simple system to study the growth and duplication of the Golgi. Cell lines stably expressing a photoactivatable GFP attached to an endogenous Golgi protein are permeabilized using digitonin. Photoactivation followed by imaging can then be used to follow the formation of the new Golgi.

Leishmania Vesicle-Depleted Exoproteome: What, Why, and How?

Leishmaniasis, a vector-borne parasitic protozoan disease, is among the most important neglected tropical diseases. In the absence of vaccines, disease management is challenging. The available chemotherapy is suboptimal, and there are growing concerns about the emergence of drug resistance. Thus, a better understanding of parasite biology is essential to generate new strategies for disease control. In this context, in vitro parasite exoproteome characterization enabled the identification of proteins involved in parasite survival, pathogenesis, and other biologically relevant processes. After 2005, with the availability of genomic information, these studies became increasingly feasible and revealed the true complexity of the parasite exoproteome. After the discovery of Leishmania extracellular vesicles (EVs), most exoproteome studies shifted to the characterization of EVs. The non-EV portion of the exoproteome, named the vesicle-depleted exoproteome (VDE), has been mostly ignored even if it accounts for a significant portion of the total exoproteome proteins. Herein, we summarize the importance of total exoproteome studies followed by a special emphasis on the available information and the biological relevance of the VDE. Finally, we report on how VDE can be studied and disclose how it might contribute to providing biologically relevant targets for diagnosis, drug, and vaccine development.

Evolution of Antigenic Variation in African Trypanosomes: Variant Surface Glycoprotein Expression, Structure, and Function

The process of antigenic variation in parasitic African trypanosomes is a remarkable mechanism for outwitting the immune system of the mammalian host, but it requires a delicate balancing act for the monoallelic expression, folding and transport of a single variant surface glycoprotein (VSG). Only one of hundreds of VSG genes is expressed at time, and this from just one of ≈15 dedicated expression sites. By switching expression of VSGs the parasite presents a continuously shifting antigenic facade leading to prolonged chronic infections lasting months to years. The basics of VSG structure and switching have been known for several decades, but recent studies have brought higher resolution to many aspects this process. New VSG structures, in silico modeling of infections, studies of VSG codon usage, and experimental ablation of VSG expression provide insights that inform how this remarkable system may have evolved.

Blocking variant surface glycoprotein synthesis alters endoplasmic reticulum exit sites/Golgi homeostasis in Trypanosoma brucei

The predominant secretory cargo of bloodstream form Trypanosoma brucei is variant surface glycoprotein (VSG), comprising ~10% total protein and forming a dense protective layer. Blocking VSG translation using Morpholino oligonucleotides triggered a precise pre‐cytokinesis arrest. We investigated the effect of blocking VSG synthesis on the secretory pathway. The number of Golgi decreased, particularly in post‐mitotic cells, from 3.5 ± 0.6 to 2.0 ± 0.04 per cell. Similarly, the number of endoplasmic reticulum exit sites (ERES) in post‐mitotic cells dropped from 3.9 ± 0.6 to 2.7 ± 0.1 eight hours after blocking VSG synthesis. The secretory pathway was still functional in these stalled cells, as monitored using Cathepsin L. Rates of phospholipid and glycosylphosphatidylinositol‐anchor biosynthesis remained relatively unaffected, except for the level of sphingomyelin which increased. However, both endoplasmic reticulum and Golgi morphology became distorted, with the Golgi cisternae becoming significantly dilated, particularly at the trans‐face. Membrane accumulation in these structures is possibly caused by reduced budding of nascent vesicles due to the drastic reduction in the total amount of secretory cargo, that is, VSG. These data argue that the total flux of secretory cargo impacts upon the biogenesis and maintenance of secretory structures and organelles in T. brucei, including the ERES and Golgi.

Surface proteins, ERAD and antigenic variation in Trypanosoma brucei

Variant surface glycoprotein (VSG) is central to antigenic variation in African trypanosomes. Although much prior work documents that VSG is efficiently synthesized and exported to the cell surface, it was recently claimed that 2-3 fold more is synthesized than required, the excess being eliminated by ER-Associated Degradation (ERAD) (Field et al., ). We now reinvestigate VSG turnover and find no evidence for rapid degradation, consistent with a model whereby VSG synthesis is precisely regulated to match requirements for a functional surface coat on each daughter cell. However, using a mutated version of the ESAG7 subunit of the transferrin receptor (E7:Ty) we confirm functional ERAD in trypanosomes. E7:Ty fails to assemble into transferrin receptors and accumulates in the ER, consistent with retention of misfolded protein, and its turnover is selectively rescued by the proteasomal inhibitor MG132. We also show that ER accumulation of E7:Ty does not induce an unfolded protein response. These data, along with the presence of ERAD orthologues in the Trypanosoma brucei genome, confirm ERAD in trypanosomes. We discuss scenarios in which ERAD could be critical to bloodstream parasites, and how these may have contributed to the evolution of antigenic variation in trypanosomes.

The endocytic activity of the flagellar pocket in Trypanosoma brucei is regulated by an adjacent phosphatidylinositol phosphate kinase

Phosphoinositides are spatially restricted membrane signaling molecules. Phosphatidylinositol 4,5-bisphosphate [PI(4,5)P2]--a phosphoinositide that is highly enriched in, and present throughout, the plasma membrane--has been implicated in endocytosis. Trypanosoma brucei has one of the highest known rates of endocytosis, a process it uses to evade the immune system. To determine whether phosphoinositides play a role in endocytosis in this organism, we have identified and characterized one of the enzymes that is responsible for generating PI(4,5)P2. Surprisingly, this phosphoinositide was found to be highly concentrated in the flagellar pocket, the only site of endocytosis and exocytosis in this organism. The enzyme (designated TbPIPKA, annotated as Tb927.10.1620) was present at the neck of the pocket, towards the anterior-end of the parasite. Depletion of TbPIPKA led to depletion of PI(4,5)P2 and enlargement of the pocket, the result of impaired endocytosis. Taken together, these data suggest that TbPIPKA and its product PI(4,5)P2 are important for endocytosis and, consequently, for homeostasis of the flagellar pocket.

Transport through the Golgi in Trypanosoma brucei

The mechanism of transport through the Golgi is still controversial, and this has led to a search for model organisms that might provide new insights. One such is the protozoan parasite, Trypanosoma brucei, which has a single Golgi whose major cargo is the GPI-anchored coat proteins that decorate the cell surface and protect the organism against immune attack through a shedding mechanism. Using published biochemical and stereological data, it is possible to show that some models for Golgi transport appear more likely than others.

The Rilp-like proteins Rilpl1 and Rilpl2 regulate ciliary membrane content

The Rilp-like proteins Rilpl1 and Rilpl2 are novel centrosomal and ciliary proteins. Depletion of Rilp-like proteins leads to the accumulation of signaling proteins in the cilium and disruption of epithelial cell organization, suggesting that Rilpl1 and Rilpl2 regulate ciliary membrane content by promoting protein removal.

The primary cilium is a microtubule-based structure found in most cell types in mammals. Disruption of cilium function causes a diverse set of human diseases collectively known as ciliopathies. We report that Rab effector–related proteins Rab-interacting lysosomal protein-like 1 (Rilpl1) and Rilpl2 regulate protein localization in the primary cilium. Rilpl2 was initially identified as up-regulated in ciliating mouse tracheal epithelial cells. Rilpl1 and Rilpl2 both localize to the primary cilium and centrosome, Rilpl1 specifically to the distal end of the mother centriole. Live-cell microscopy reveals that Rilpl2 primary cilium localization is dynamic and that it is associated with tubulovesicular structures at the base of the cilium. Depletion of Rilpl1 and Rilpl2 results in accumulation of signaling proteins in the ciliary membrane and prevents proper epithelial cell organization in three-dimensional culture. These data suggest that Rilp-like proteins function in regulation of ciliary membrane protein concentration by promoting protein removal from the primary cilium.

CdTe and CdSe quantum dots cytotoxicity: a comparative study on microorganisms

Quantum dots (QDs) are colloidal semiconductor nanocrystals of a few nanometers in diameter, being their size and shape controlled during the synthesis. They are synthesized from atoms of group II–VI or III–V of the periodic table, such as cadmium telluride (CdTe) or cadmium selenium (CdSe) forming nanoparticles with fluorescent characteristics superior to current fluorophores. The excellent optical characteristics of quantum dots make them applied widely in the field of life sciences. Cellular uptake of QDs, location and translocation as well as any biological consequence, such as cytotoxicity, stimulated a lot of scientific research in this area. Several studies pointed to the cytotoxic effect against micoorganisms. In this mini-review, we overviewed the synthesis and optical properties of QDs, and its advantages and bioapplications in the studies about microorganisms such as protozoa, bacteria, fungi and virus.

Differential selection of Golgi proteins by COPII Sec24 isoforms in procyclic Trypanosoma brucei

The Sec24 subunit of the coat protein complex II (COPII) has been implicated in selecting newly synthesized cargo from the endoplasmic reticulum (ER) for delivery to the Golgi. The protozoan parasite, Trypanosoma brucei, contains two paralogs, TbSec24.1 and TbSec24.2, which were depleted using RNA interference in the insect form of the parasite. Depletion of either TbSec24.1 or TbSec24.2 resulted in growth arrest and modest inhibition of anterograde transport of the putative Golgi enzyme, TbGntB, and the secretory marker, BiPNAVRG-HA9. In contrast, depletion of TbSec24.1, but not TbSec24.2, led to reversible mislocalization of the Golgi stack proteins, TbGRASP and TbGolgin63. The latter accumulated in the ER. The localization of the COPI coatomer subunit, TbεCOP, and the trans Golgi network (TGN) protein, TbGRIP70, was largely unaffected, although the latter was preferentially lost from those Golgi that were not associated with the bilobe, a structure previously implicated in Golgi biogenesis. Together, these data suggest that TbSec24 paralogs can differentiate among proteins destined for the Golgi.

Unraveling the Golgi ribbon

The Golgi apparatus lies at the heart of the secretory pathway where it receives, modifies and sorts protein cargo to the proper intracellular or extracellular location. Although this secretory function is highly conserved throughout the eukaryotic kingdom, the structure of the Golgi complex is arranged very differently among species. In particular, Golgi membranes in vertebrate cells are integrated into a single compact entity termed the Golgi ribbon that is normally localized in the perinuclear area and in close vicinity to the centrosomes. This organization poses a challenge for cell division when the single Golgi ribbon needs to be partitioned into the two daughter cells. To ensure faithful inheritance in the progeny, the Golgi ribbon is divided in three consecutive steps in mitosis, namely disassembly, partitioning and reassembly. However, the structure of the Golgi ribbon is only present in higher animals and Golgi disassembly during mitosis is not ubiquitous in all organisms. Therefore, there must be unique reasons to build up the Golgi in this particular conformation and to preserve it over generations. In this review, we first highlight the diversity of the Golgi architecture in different organisms and revisit the concept of the Golgi ribbon. Following on, we discuss why the ribbon is needed and how it forms in vertebrate cells. Lastly, we conclude with likely purposes of mitotic ribbon disassembly and further propose mechanisms by which it regulates mitosis.

DEAD-box RNA helicase is dispensable for mitochondrial translation in Trypanosoma brucei

DEAD-box RNA helicase, a putative subunit of the mitochondrial ribosome of Trypanosoma brucei, has been down-regulated in the procyclic and bloodstream stage by RNA interference. Although ablation of the transcript leads to a week growth phenotype in the procyclic cells, the protein does not seem to be essential for mitochondrial translation under standard cultivation conditions, as shown by an assay that allows visualization of the de novo synthesized proteins. Furthermore, we show that synthesis of cytochrome c oxidase subunit I and cytochrome b does not occur in the mitochondrion of the bloodstream stage.

Chaperone requirements for biosynthesis of the trypanosome variant surface glycoprotein

Background

Trypanosoma brucei does not respond transcriptionally to several endoplasmic reticulum (ER) stress conditions, including tunicamycin or dithiothreitol, indicating the absence of a conventional unfolded protein response. This suggests divergent mechanisms for quality control (QC) of ER protein folding and export may be present in trypanosomes. As the variant surface glycoprotein (VSG) represents ∼90% of trypanosome plasma membrane protein, it is possible that VSG has evolved to fold efficiently to minimize ER folding burden.

Methodology/Principal Findings

We demonstrate the presence of a QC system by pharmacological inhibition of the trypanosome 26S proteasome. This indicates active proteasome-mediated VSG turnover as ∼2.5 fold more VSG is recovered from cell lysates following MG132 inhibition. An in silico scan of the trypanosome genome identified 28 open reading frames likely to encode polypeptides participating in ER nascent chain maturation. By RNA interference we monitored the importance of these gene products to proliferation, VSG abundance and cell morphology. 68% of the cohort were required for normal proliferation, and depletion of most of these factors resulted in increased VSG abundance, suggesting involvement in ERQC and degradation.

Conclusions/Significance

The retention of genes for, and the involvement of many gene products in, VSG folding indicates a substantial complexity within the pathways required to perform this role. Counterintuitively, for a super-abundant antigen VSG is apparently made in excess. The biosynthetic excess VSG appears to be turned over efficiently by the proteasome, implying that considerable VSG is rejected by the trypanosome ERQC mechanism. Accordingly, the VSG polypeptide is not well optimized for folding, as only ∼30% attains the native state. Finally as much of the core ERQC system is functionally conserved in trypanosomes, the pathway has an ancient evolutionary origin, and was present in the last common eukaryotic ancestor.

Streamlined architecture and glycosylphosphatidylinositol-dependent trafficking in the early secretory pathway of African trypanosomes

The variant surface glycoprotein (VSG) of bloodstream form Trypanosoma brucei (Tb) is a critical virulence factor. The VSG glycosylphosphatidylinositol (GPI)-anchor strongly influences passage through the early secretory pathway. Using a dominant-negative mutation of TbSar1, we show that endoplasmic reticulum (ER) exit of secretory cargo in trypanosomes is dependent on the coat protein complex II (COPII) machinery. Trypanosomes have two orthologues each of the Sec23 and Sec24 COPII subunits, which form specific heterodimeric pairs: TbSec23.1/TbSec24.2 and TbSec23.2/TbSec24.1. RNA interference silencing of each subunit is lethal but has minimal effects on trafficking of soluble and transmembrane proteins. However, silencing of the TbSec23.2/TbSec24.1 pair selectively impairs ER exit of GPI-anchored cargo. All four subunits colocalize to one or two ER exit sites (ERES), in close alignment with the postnuclear flagellar adherence zone (FAZ), and closely juxtaposed to corresponding Golgi clusters. These ERES are nucleated on the FAZ-associated ER. The Golgi matrix protein Tb Golgi reassembly stacking protein defines a region between the ERES and Golgi, suggesting a possible structural role in the ERES:Golgi junction. Our results confirm a selective mechanism for GPI-anchored cargo loading into COPII vesicles and a remarkable degree of streamlining in the early secretory pathway. This unusual architecture probably maximizes efficiency of VSG transport and fidelity in organellar segregation during cytokinesis.

Ultrastructural study of Golgi duplication in Trypanosoma brucei

Golgi duplication in the protozoan parasite Trypanosoma brucei has been tracked using serial thin section three-dimensional reconstructions of transmission electron micrographs. The old Golgi maintains a constant size (approximately 0.060 microm(3)) throughout the cell cycle. A morphologically identifiable new Golgi appears at approximately 0.20 of the cell cycle (defined by the size of the nucleus and lasting about 9 h) and grows from approximately 0.018 microm(3) until it is the same size as the old Golgi (by approximately 0.55 of the cell cycle). Morphologically identifiable late Golgi appear at approximately 0.58 of the cell cycle, but their volume ( approximately 0.036 microm(3)) did not change significantly. Cryoimmunoelectron microscopy was used to identify candidates for the earliest new Golgi structures, and these comprised clusters of vesicles containing Golgi reassembly stacking protein (GRASP) near an endoplasmic reticulum exit site. These results, combined with earlier fluorescence data, suggest that the new Golgi begins functioning before cisternal stacks are formed.

Polo-like kinase is required for Golgi and bilobe biogenesis in Trypanosoma brucei

A bilobed structure marked by TbCentrin2 regulates Golgi duplication in the protozoan parasite Trypanosoma brucei. This structure must itself duplicate during the cell cycle for Golgi inheritance to proceed normally. We show here that duplication of the bilobed structure is dependent on the single polo-like kinase (PLK) homologue in T. brucei (TbPLK). Depletion of TbPLK leads to malformed bilobed structures, which is consistent with an inhibition of duplication and an increase in the number of dispersed Golgi structures with associated endoplasmic reticulum exit sites. These data suggest that the bilobe may act as a scaffold for the controlled assembly of the duplicating Golgi.

TbG63, a golgin involved in Golgi architecture in Trypanosoma brucei

Golgins are coiled-coil proteins that have been implicated in the structure and function of the Golgi complex. Here, we identify and characterize a trypanosomal golgin, TbG63, showing that it has a C-terminal membrane anchor and an N-terminus that projects into the cytoplasm. TbG63 in procyclic parasites is localized to the Golgi and interacts with the active, GTP-form of TbRab1A. Overexpression of TbG63 has dramatic effects on Golgi architecture -- effects that require the N-terminus -- whereas depletion has little, if any, effect on the growth rate. By contrast, in the bloodstream form of the parasite, depletion of TbG63 slows growth, although it has no obvious effect on the transport of a variant surface glycoprotein (VSG) or on Golgi structure. TbG63 might be a useful tool to study the structure and functioning of the Golgi complex.

Crithidia deanei: influence of parasite gp63 homologue on the interaction of endosymbiont-harboring and aposymbiotic strains with Aedes aegypti midgut

The present study demonstrates that the endosymbiont of Crithidia deanei influences the expression of surface gp63 molecules. Ultrastructural immunocytochemical analysis shows the presence of the gp63-like protein in the protozoan flagellum and flagellar pocket, either attached to shed membranes or in a free form. This molecule is glycosylphosphatidylinositol (GPI) anchored to the plasma membrane as demonstrated by phospholipase C (PLC) treatment and cross-reacting determinant detection by immunoblotting. The gp63 molecule mediates the adhesive process of the protozoan to Aedes aegypti explanted guts, since the binding was reduced by pre-incubating the C. deanei parasites (wild and aposymbiotic strains) with anti-gp63 antibodies, PLC or PLC followed by anti-gp63 antibodies incubation. In addition, the number of wild C. deanei bound to A. aegypti explanted guts was twice as that of aposymbiotic parasites. Flow cytometry assays revealed that the reactivity of the wild strain with anti-gp63 antibodies was approximately twice as that of the aposymbiotic strain. We may conclude that higher expression of surface gp63 by the wild strain of C. deanei may positively influence this interaction, posing a prominent advantage for the endosymbiont-containing trypanosomatids.

Intracellular Trafficking in the Trypanosomatids

Trypanosomes are members of the kinetoplastida, a group of divergent protozoan parasites responsible for considerable morbidity and mortality worldwide. These organisms have highly complex life cycles requiring modification of their cell surface together with engagement of immune evasion systems to effect survival; both processes intimately involve the membrane trafficking system. The completion of three trypanosomatid and several additional protist genomes in the last few years is providing an exciting opportunity to evaluate, at the molecular level, the evolution and diversity of membrane trafficking across deep evolutionary time as well as to analyse in unprecedented detail the membrane trafficking systems of trypanosomes.

De novo sphingolipid synthesis is essential for viability, but not for transport of glycosylphosphatidylinositol-anchored proteins, in African trypanosomes

De novo sphingolipid synthesis is required for the exit of glycosylphosphatidylinositol (GPI)-anchored membrane proteins from the endoplasmic reticulum in yeast. Using a pharmacological approach, we test the generality of this phenomenon by analyzing the transport of GPI-anchored cargo in widely divergent eukaryotic systems represented by African trypanosomes and HeLa cells. Myriocin, which blocks the first step of sphingolipid synthesis (serine + palmitate --> 3-ketodihydrosphingosine), inhibited the growth of cultured bloodstream parasites, and growth was rescued with exogenous 3-ketodihydrosphingosine. Myriocin also blocked metabolic incorporation of [3H]serine into base-resistant sphingolipids. Biochemical analyses indicate that the radiolabeled lipids are not sphingomyelin or inositol phosphorylceramide, suggesting that bloodstream trypanosomes synthesize novel sphingolipids. Inhibition of de novo sphingolipid synthesis with myriocin had no adverse effect on either general secretory trafficking or GPI-dependent trafficking in trypanosomes, and similar results were obtained with HeLa cells. A mild effect on endocytosis was seen for bloodstream trypanosomes after prolonged incubation with myriocin. These results indicate that de novo synthesis of sphingolipids is not a general requirement for secretory trafficking in eukaryotic cells. However, in contrast to the closely related kinetoplastid Leishmania major, de novo sphingolipid synthesis is essential for the viability of bloodstream-stage African trypanosomes.

Ordered assembly of the duplicating Golgi in Trypanosoma brucei

The new Golgi in the protozoan parasite Trypanosoma brucei grows near to the old and adjacent to the growing new endoplasmic reticulum exit site. Growth is now shown to be at least a two-stage process, in which a representative matrix marker (GRASP) and enzyme (GntB) are delivered to the site of assembly, followed approximately 10 min later by a COPI component (epsilon-COP) and a trans-Golgi network (TGN) marker (GRIP70). A secretory cargo marker (signal sequence-YFP) appeared early near the new endoplasmic reticulum exit site but did not enter the Golgi until the second stage. Together these data suggest that structural and enzymatic components of the new Golgi stack are laid down first, followed by those needed to move and sort the cargo passing through it.

GPI valence and the fate of secretory membrane proteins in African trypanosomes

Progression of GPI-anchored proteins in bloodstream African trypanosomes correlates with GPI-valence: homodimeric VSG (2 GPI) is a surface protein; heterodimeric transferrin receptor (1 GPI) localizes in the flagellar pocket; homodimeric GPI-minus VSG (0 GPI) is rapidly degraded in the lysosome. We test this relationship using three native secretory/endocytic proteins as monomeric GPI-plus and -minus reporters. GPI-minus procyclin trafficks to the lysosome and is degraded. GPI-plus procyclin trafficks to the flagellar pocket/cell surface and is released (∼50%) with an intact anchor, the remainder (∼50%) is degraded in the lysosome. GPI-plus BiPNHP, derived from the ER marker BiP, is released quantitatively (>80%), while GPI-plus p67HP, derived from the lysosomal marker p67, turns over by both release (∼15%) and lysosomal degradation (>50%). Turnover of endogenous transferrin receptor occurs primarily by lysosomal degradation (>90%). Thus shedding of monovalent GPI reporters correlates inversely with lysosomal targeting. We propose that mono-GPI reporters cycle through the flagellar pocket and endosome until they are disposed of by either shedding or lysosomal targeting. Partitioning between these fates may be a function of individual physical properties. Release is likely due to the exclusive use of C-14:0 myristate in the bloodstream stage GPI anchor. Up-regulation of transferrin receptor by culture in dog serum resulted in prominent cell surface localization, but not in elevated release. Surface receptor was non-functional for ligand binding suggesting that it may be bivalent homodimers of the GPI-anchored ESAG6 receptor subunit.

Functional analysis of TbARL1, an N-myristoylated Golgi protein essential for viability in bloodstream trypanosomes

Myristoyl-CoA:protein N-myristoyltransferase (NMT), an essential protein in Trypanosoma brucei and Leishmania major, catalyses the covalent attachment of the fatty acid myristate to the N-terminus of a range of target proteins. In order to define the essential targets contributing to lethality in the absence of NMT activity, we have focused on the ADP-ribosylation factor (Arf) family of GTP-binding proteins, as growth arrest in Saccharomyces cerevisiae mutants with reduced NMT activity correlates with a decrease in N-myristoylated Arf proteins. We have identified nine Arf/Arls in the T. brucei and T. cruzi genomes and ten in L. major. Characterization of the T. brucei ARL1 homologue has revealed that the protein is localized in the Golgi apparatus and is expressed only in the mammalian bloodstream form of the parasite and not in the insect procyclic stage. This is the only reported example to date of a differentially expressed ARL1 homologue in any species. We have used RNA interference to demonstrate that ARL1 is essential for viability in T. brucei bloodstream parasites. Prior to cell death, depletion of ARL1 protein in bloodstream parasites results in abnormal morphology, including disintegration of the Golgi structure, multiple flagella and nuclei, and the presence of large numbers of vesicles. The cells have only a minor apparent defect in endocytosis but exocytosis of variant surface glycoprotein to the parasite surface is significantly delayed. RNA interference of ARL1 in procyclic cells has no effect on parasite growth or morphology. Our results suggest that there may be different pathways regulating Golgi structure and function in the two major life cycle stages of T. brucei.

Golgi duplication in Trypanosoma brucei

Duplication of the single Golgi apparatus in the protozoan parasite Trypanosoma brucei has been followed by tagging a putative Golgi enzyme and a matrix protein with variants of GFP. Video microscopy shows that the new Golgi appears de novo, near to the old Golgi, about two hours into the cell cycle and grows over a two-hour period until it is the same size as the old Golgi. Duplication of the endoplasmic reticulum (ER) export site follows exactly the same time course. Photobleaching experiments show that the new Golgi is not the exclusive product of the new ER export site. Rather, it is supplied, at least in part, by material directly from the old Golgi. Pharmacological experiments show that the site of the new Golgi and ER export is determined by the location of the new basal body.

Glycosylphosphatidylinositol-dependent protein trafficking in bloodstream stage Trypanosoma brucei

We have previously demonstrated that glycosylphosphatidylinositol (GPI) anchors strongly influence protein trafficking in the procyclic insect stage of Trypanosoma brucei (M. A. McDowell, D. A. Ransom, and J. D. Bangs, Biochem. J. 335:681-689, 1998), where GPI-minus variant surface glycoprotein (VSG) reporters have greatly reduced rates of endoplasmic reticulum (ER) exit but are ultimately secreted. We now demonstrate that GPI-dependent trafficking also occurs in pathogenic bloodstream trypanosomes. However, unlike in procyclic trypanosomes, truncated VSGs lacking C-terminal GPI-addition signals are not secreted but are mistargeted to the lysosome and degraded. Failure to export these reporters is not due to a deficiency in secretion of these cells since the N-terminal ATPase domain of the endogenous ER protein BiP is efficiently secreted from transgenic cell lines. Velocity sedimentation experiments indicate that GPI-minus VSG dimerizes similarly to wild-type VSG, suggesting that degradation is not due to ER quality control mechanisms. However, GPI-minus VSGs are fully protected from degradation by the cysteine protease inhibitor FMK024, a potent inhibitor of the major lysosomal protease trypanopain. Immunofluorescence of cells incubated with FMK024 demonstrates that GPI-minus VSG colocalizes with p67, a lysosomal marker. These data suggest that in the absence of a GPI anchor, VSG is mistargeted to the lysosome and subsequently degraded. Our findings indicate that GPI-dependent transport is a general feature of secretory trafficking in both stages of the life cycle. A working model is proposed in which GPI valence regulates progression in the secretory pathway of bloodstream stage trypanosomes.

The kinetoplastida endocytic apparatus. Part I: a dynamic system for nutrition and evasion of host defences

The endocytic system of kinetoplastid parasites is a highly polarized membrane network focused on the flagellar pocket localized at one end of the cell. When first characterized, the endosomal network was envisioned as a simple system for uptake of extracellular material by fluid-phase or receptor-mediated mechanisms. Subsequently, it has become clear that the kinetoplastid endosomal system has an active and vital role in avoiding the host immune system and virulence, as well as providing the basic functions to fulfil cellular nutritional requirements. In two reviews, recent advances in the definition and comprehension of kinetoplastida endocytosis are discussed and, in Trypanosoma brucei in particular as the more developed experimental system. In Part 1, the endocytic system is considered in context of the surface molecules and their potential roles in virulence.

Targeting of the GRIP domain to the trans-Golgi network is conserved from protists to animals

The GRIP domain, found in a family of coiled-coil peripheral membrane Golgi proteins, is a specific targeting sequence for the trans-Golgi network of animal cells. In this study we show that a coiled-coil protein with a GRIP domain occurs in the primitive eukaryote, Trypanosoma brucei, and that reporter proteins containing this domain can be used as a marker for the poorly characterized trans Golgi/trans-Golgi network of trypanosomatid parasites. The T. brucei GRIP domain, when fused to the carboxyl terminus of the green fluorescent protein (GFP-TbGRIP), was efficiently localized to the Golgi apparatus of transfected COS cells. Overexpression of GFP-TbGRIP in COS cells displaced the endogenous GRIP protein, GCC1p, from the Golgi apparatus indicating that the trypanosomatid and mammalian GRIP sequences interact with similar membrane determinants. GFP fusion proteins containing either the T. brucei GRIP domain or the human p230 GRIP (p230GRIP) domain were also expressed in the trypanosomatid parasite, Leishmania mexicana, and localized by fluorescence and immuno-electron microscopy to the trans face of the single Golgi apparatus and a short tubule that extended from the Golgi apparatus. Binding of GFP-p230GRIP to Golgi membranes in L. mexicana was abrogated by mutation of a critical tyrosine residue in the p230 GRIP domain. The levels of GFP-GRIP fusion proteins were dramatically reduced in stationary-phase L. mexicana promastigotes, suggesting that specific Golgi trafficking steps may be down-regulated as the promastigotes cease dividing. This study provides a protein marker for the trans-Golgi network of trypanosomatid parasites and suggests that the GRIP domain binds to a membrane component that has been highly conserved in eukaryotic evolution.

Accumulation of a GPI-anchored protein at the cell surface requires sorting at multiple intracellular levels

Proteins modified by glycosylphosphatidylinositol membrane anchors have become popular for investigating the role of membrane lipid microdomains in cellular sorting processes. To this end, trypanosomatids offer the advantage that they express these molecules in high abundance. The parasitic protozoan Trypanosoma brucei is covered by a dense and nearly homogeneous coat composed of a glycosylphosphatidylinositol-anchored protein, the variant surface glycoprotein, which is essential for survival of the parasite in the mammalian blood. Therefore, T. brucei must possess mechanisms to selectively and efficiently deliver variant surface glycoprotein to the cell surface. In this study, we have quantified the steady-state distribution of variant surface glycoprotein by differential biotinylation, by fluorescence microscopy and by immunoelectron microscopy on high-pressure frozen and freeze-substituted samples. These three techniques provide very similar estimates of the fraction of variant surface glycoprotein located on the cell surface, on average 89.4%. The intracellular variant surface glycoprotein (10.6%) is predominantly located in the endosomal compartment (75%), while 25% are associated with the endoplasmic reticulum, Golgi apparatus and lysosomes. The density of variant surface glycoprotein in the plasma membrane including the membrane of the flagellar pocket, the only site for endo- and exocytosis in this organism, is 48-52 times higher than the density in endoplasmic reticulum membranes. The relative densities of the Golgi complex and of the endosomes are 2.7 and 10.8, respectively, compared to the endoplasmic reticulum. This data set provides the basis for an analysis of the dynamics of sorting. Depending on the intracellular itinerary of newly formed variant surface glycoprotein, the high surface density is achieved in two (endoplasmic reticulum --> Golgi complex --> cell surface) or three enrichment steps (endoplasmic reticulum --> Golgi complex --> endosomes --> cell surface), suggesting sorting between several membrane compartments.

GPI anchor transamidase of Trypanosoma brucei: in vitro assay of the recombinant protein and VSG anchor exchange

GPI8 from Trypanosoma brucei was cloned and expressed in Escherichia coli. TbGPI8 encodes a 37 kDa protein (35 kDa after removal of the putative signal sequence) with a pI of 5.5. It contains one potential N-glycosylation site near the N-terminus but no C-terminal hydrophobic region. Enzyme activity assays using trypanosomal lysates or recombinant TbGpi8 exhibited cleavage of the synthetic peptide acetyl-S-V-L-N-aminomethyl-coumarine, indicating that TbGpi8 is indeed directly involved in the proteolytic processing of the GPI anchoring signal. Intracellular localization of TbGpi8 within tubular structures, such as the endoplasmic reticulum, was observed by using specific anti-TbGpi8 antibodies.

The transamidase mechanism of GPI anchoring was studied in bloodstream forms of Trypanosoma brucei using media containing hydrazine or biotinylated hydrazine. In the presence of the latter nucleophile, part of the newly formed VSG was linked to this instead of the GPI anchor and was not transferred to the cell surface. VSG-hydrazine-biotin was detected by streptavidin in western blots and intracellularly in Golgi-like compartments.

Intracellular trafficking of glycosylphosphatidylinositol (GPI)-anchored proteins and free GPIs in Leishmania mexicana

Free glycosylphosphatidylinositols (GPIs) are an important class of membrane lipids in many pathogenic protozoa. In this study, we have investigated the subcellular distribution and intracellular trafficking of an abundant class of free GPIs [termed glycosylinositolphospholipids (GIPLs)] in Leishmania mexicana promastigotes. The intracellular transport of the GIPLs and the major GPI-anchored glycoprotein gp63 was measured by following the incorporation of these molecules into sphingolipid-rich, detergent-resistant membranes (DRMs) in the plasma membrane. In metabolic-labelling experiments, mature GIPLs and gp63 were transported to DRMs in the plasma membrane with a t(1/2) of 70 and 40 min, respectively. Probably, GIPL transport to the DRMs involves a vesicular mechanism, as transport of both the GIPLs and gp63 was inhibited similarly at 10 degrees C. All GIPL intermediates were quantitatively recovered in Triton X-100-soluble membranes and were largely orientated on the cytoplasmic face of the endoplasmic reticulum, as shown by their sensitivity to exogenous phosphatidylinositol-specific phospho-lipase C. On the contrary, a significant proportion of the mature GIPLs ( approximately 50% of iM4) were accessible to membrane-impermeable probes on the surface of live promastigotes. These results suggest that the GIPLs are flipped across intracellular or plasma membranes during surface transport and that a significant fraction may populate the cytoplasmic leaflet of the plasma membrane. Finally, treatment of L. mexicana promastigotes with myriocin, an inhibitor of sphingolipid biosynthesis, demonstrated that ongoing sphingolipid biosynthesis is not required for the plasma-membrane transport of either gp63 or the GIPLs and that DRMs persist even when cellular levels of the major sphingolipid are depleted by 70%.

Secretory pathway of trypanosomatid parasites

The Trypanosomatidae comprise a large group of parasitic protozoa, some of which cause important diseases in humans. These include Trypanosoma brucei (the causative agent of African sleeping sickness and nagana in cattle), Trypanosoma cruzi (the causative agent of Chagas' disease in Central and South America), and Leishmania spp. (the causative agent of visceral and [muco]cutaneous leishmaniasis throughout the tropics and subtropics). The cell surfaces of these parasites are covered in complex protein- or carbohydrate-rich coats that are required for parasite survival and infectivity in their respective insect vectors and mammalian hosts. These molecules are assembled in the secretory pathway. Recent advances in the genetic manipulation of these parasites as well as progress with the parasite genome projects has greatly advanced our understanding of processes that underlie secretory transport in trypanosomatids. This article provides an overview of the organization of the trypanosomatid secretory pathway and connections that exist with endocytic organelles and multiple lytic and storage vacuoles. A number of the molecular components that are required for vesicular transport have been identified, as have some of the sorting signals that direct proteins to the cell surface or organelles in the endosome-vacuole system. Finally, the subcellular organization of the major glycosylation pathways in these parasites is reviewed. Studies on these highly divergent eukaryotes provide important insights into the molecular processes underlying secretory transport that arose very early in eukaryotic evolution. They also reveal unusual or novel aspects of secretory transport and protein glycosylation that may be exploited in developing new antiparasite drugs.

The flagellum and flagellar pocket of trypanosomatids

The flagellum and flagellar pocket are distinctive organelles present among all of the trypanosomatid protozoa. Currently, recognized functions for these organelles include generation of motility for the flagellum and dedicated secretory and endocytic activities for the flagellar pocket. The flagellar and flagellar pocket membranes have long been recognized as morphologically separate domains that are component parts of the plasma membrane that surrounds the entire cell. The structural and functional specialization of these two membranes has now been underscored by the identification of multiple proteins that are targeted selectively to each of these domains, and non-membrane proteins have also been identified that are targeted to the internal lumina of these organelles. Investigations on the functions of these organelle-specific proteins should continue to shed light on the unique biological activities of the flagellum and flagellar pocket. In addition, work has begun on identifying signals or modifications of these proteins that direct their targeting to the correct subcellular location. Future endeavors should further refine our knowledge of targeting signals and begin to dissect the molecular machinery involved in transporting and retaining each polypeptide at its designated cellular address.

Surface coats and secretory trafficking in African trypanosomes

Recent advances in transfection technology have been exploited to address fundamental questions relating to secretory trafficking in African trypanosomes. Targeted gene disruptions and ectopic expression of the major stage-specific surface proteins have provided unexpected insights into both the function and assembly of the essential parasite surface coats. A growing list of novel secretory cargo molecules, as well as advances in the characterization of trypanosomal secretory machinery, provide a unique model system for the study of eukaryotic secretory processes.

Both IgM and IgG anti-VSG antibodies initiate a cycle of aggregation-disaggregation of bloodstream forms of Trypanosoma brucei without damage to the parasite

Bloodstream forms of Trypanosoma brucei, when aggregated in the presence of either acute immune plasma, acute immune serum, purified IgM anti-VSG antibodies or purified IgG anti-VSG antibodies, subsequently disaggregated with a t1/2 for disaggregation of 15 min at 37 degrees C as long as the trypanosomes were metabolically active at the beginning of the experiment and maintained during the experiment in a suitable supporting medium. The t1/2 for disaggregation was found to be directly dependent upon temperature and inversely proportional to the antibody concentration. The trypanosomes were always motile and metabolically active during aggregation and after disaggregation and were fully infective for a mammalian host following disaggregation as well as able to grow and divide normally during axenic culture. The disaggregation was strictly energy dependent and was inhibited when intracellular ATP levels were reduced by salicylhydroxamic acid or following addition of oligomycin while respiring glucose. In addition the process of disaggregation was dependent upon normal endosomal activity as evidenced by its sensitivity to a wide variety of inhibitors of various endosomal functions. Disaggregation was not due to separation of immunoglobulin chains by either disulphide reduction or disulphide exchange reactions and gross proteolytic cleavage of the immunoglobulins attached to the surface of the parasite was not detected. In addition, gross cleavage or release of the VSG from the surface of the cell did not occur during disaggregation but proteolytic cleavage of a small proportion of either the VSG or the immunoglobulins could not be eliminated from consideration. Finally the mechanism of disaggregation was found to be a regulated process, independent of Ca2+ movements but dependent upon the activity of protein kinase C or related kinases and inhibited by the activity of protein kinase A as evidenced by the effects of a panel of inhibitors and cAMP analogues on the process of disaggregation. The mechanism of disaggregation displayed by trypanosomes aggregated by anti-VSG antibody is proposed to form part of the parasite's defence against the host immune system and functions to aid survival of trypanosomes in the presence of antibody in the host prior to the occurrence of a VSG switching event.

Expression of bloodstream variant surface glycoproteins in procyclic stage Trypanosoma brucei: role of GPI anchors in secretion

Using transformed procyclic trypanosomes, the synthesis, intracellular transport and secretion of wild-type and mutant variant surface glycoprotein (VSG) is characterized. We find no impediment to the expression of this bloodstream stage protein in insect stage cells. VSG receives a procyclic-type phosphatidylinositol-specific phospholipase C-resistant glycosyl phosphatidylinositol (GPI) anchor, dimerizes and is N-glycosylated. It is transported to the plasma membrane with rapid kinetics (t(1/2) approximately 1 h) and then released by a cell surface zinc-dependent metalloendoprotease activity, a possible homolog of leishmanial gp63. Deletion of the C-terminal GPI addition signal generates a soluble form of VSG that is exported with greatly reduced kinetics (t(1/2) approximately 5 h). Fusion of the procyclic acidic repetitive protein (PARP) GPI anchor signal to the C-terminus of the truncated VSG reporter restores both GPI addition and transport competence, suggesting that GPI anchors play a critical role in the folding and/or forward transport of newly synthesized VSG. The VSG-PARP fusion is also processed near the C-terminus by events that do not involve N-linked oligosaccharides and which are consistent with GPI side chain modification. This unexpected result suggests that GPI processing may be influenced by adjacent peptide sequence or conformation.

Endocytosis and secretion in trypanosomatid parasites — Tumultuous traffic in a pocket

Trypanosomatids are flagellated protozoan parasites of invertebrates, vertebrates and plants. Some species, found in the subtropics and tropics, cause chronic diseases in humans and domestic animals. The surface of the trypanosomatid provides a shield against environmental challenges, ligands for interaction with host cells, as well as receptors and transporters for the uptake of nutrients. Communication between the parasite and its environment is confined to the flagellar pocket, an invagination of the plasma membrane around the base of the flagellum. In this review, the authors discuss endocytosis, secretion and membrane trafficking in Trypanosoma and Leishmania.

The secretory pathway of protists: spatial and functional organization and evolution

All cells secrete a diversity of macromolecules to modify their environment or to protect themselves. Eukaryotic cells have evolved a complex secretory pathway consisting of several membrane-bound compartments which contain specific sets of proteins. Experimental work on the secretory pathway has focused mainly on mammalian cell lines or on yeasts. Now, some general principles of the secretory pathway have become clear, and most components of the secretory pathway are conserved between yeast cells and mammalian cells. However, the structure and function of the secretory system in protists have been less extensively studied. In this review, we summarize the current knowledge about the secretory pathway of five different groups of protists: Giardia lamblia, one of the earliest lines of eukaryotic evolution, kinetoplastids, the slime mold Dictyostelium discoideum, and two lineages within the "crown" of eukaryotic cell evolution, the alveolates (ciliates and Plasmodium species) and the green algae. Comparison of these systems with the mammalian and yeast system shows that most elements of the secretory pathway were presumably present in the earliest eukaryotic organisms. However, one element of the secretory pathway shows considerable variation: the presence of a Golgi stack and the number of cisternae within a stack. We suggest that the functional separation of the plasma membrane from the nucleus-endoplasmic reticulum system during evolution required a sorting compartment, which became the Golgi apparatus. Once a Golgi apparatus was established, it was adapted to the various needs of the different organisms.

Cortical structure and function in euglenoids with reference to trypanosomes, ciliates, and dinoflagellates

The membrane skeletal complex (cortex) of euglenoids generates and maintains cell form. In this review we summarize structural, biochemical, physiological, and molecular studies on the euglenoid membrane skeleton, focusing specifically on four principal components: the plasma membrane, a submembrane layer (epiplasm), cisternae of the endoplasmic reticulum, and microtubules. The data from euglenoids are compared with findings from representative organisms of three other protist groups: the trypanosomes, ciliates, and dinoflagellates. Although there are significant differences in cell form and phylogenetic affinities among these groups, there are also many similarities in the organization and possibly the function of their cortical components. For example, an epiplasmic (membrane skeletal) layer is widely used for adding strength and rigidity to the cell surface. The ER/alveolus/amphiesmal vesicle may function in calcium storage and regulation, and in mediating assembly of surface plates. GPI-linked variable surface antigens are characteristic of both ciliates and the unrelated trypanosomatids. Microtubules are ubiquitous, and cortices in trypanosomes may relay exclusively on microtubules and microtubule-associated proteins for maintaining cell form. Also, in agreement with previous suggestions, there is an apparent preservation of many cortical structures during cell duplication. In three of the four groups there is convincing evidence that part or all of the parental cortex persists during cytokinesis, thereby producing mosaics or chimeras consisting of both inherited and newly synthesized cortical components.

Identification of six complementation classes involved in the biosynthesis of glycosylphosphatidylinositol anchors in Saccharomyces cerevisiae

Glycosylphosphatidylinositol (GPI)-anchored membrane proteins are synthesized by the posttranslational attachment of a preformed glycolipid to newly made glycoproteins. alpha-Agglutinin is a GPI-anchored glycoprotein that gets expressed at the cell surface of MAT alpha cells after induction with type a mating factor. Mutants affecting the biosynthesis of GPI anchors were obtained by selecting for the absence of alpha-agglutinin from the cell wall after induction with a-factor at 37 degrees C. 10 recessive mutants were grouped into 6 complementation classes, gpi4 to gpi9. Mutants are considered to be deficient in the biosynthesis of GPI anchors, since each mutant accumulates an abnormal, incomplete GPI glycolipid containing either zero, two, or four mannoses. One mutant accumulates a complete precursor glycolipid, suggesting that it might be deficient in the transfer of complete precursor lipids to proteins. When labeled with [2-3H]inositol, mutants accumulate reduced amounts of radiolabeled GPI-anchored proteins, and the export of the GPI-anchored Gas1p out of the ER is severely delayed in several mutant strains. On the other hand, invertase and acid phosphatase are secreted by all but one mutant. All mutants show an increased sensitivity to calcofluor white and hygromycin B. This suggests that GPI-anchored proteins are required for the integrity of the yeast cell wall.

Protein trafficking in kinetoplastid protozoa

The kinetoplastid protozoa infect hosts ranging from invertebrates to plants and mammals, causing diseases of medical and economic importance. They are the earliest-branching organisms in eucaryotic evolution to have either mitochondria or peroxisome-like microbodies. Investigation of their protein trafficking enables us to identify characteristics that have been conserved throughout eucaryotic evolution and also reveals how far variations, or alternative mechanisms, are possible. Protein trafficking in kinetoplastids is in many respects similar to that in higher eucaryotes, including mammals and yeasts. Differences in signal sequence specificities exist, however, for all subcellular locations so far examined in detail--microbodies, mitochondria, and endoplasmic reticulum--with signals being more degenerate, or shorter, than those of their higher eucaryotic counterparts. Some components of the normal array of trafficking mechanisms may be missing in most (if not all) kinetoplastids: examples are clathrin-coated vesicles, recycling receptors, and mannose 6-phosphate-mediated lysosomal targeting. Other aspects and structures are unique to the kinetoplastids or are as yet unexplained. Some of these peculiarities may eventually prove to be weak points that can be used as targets for chemotherapy; others may turn out to be much more widespread than currently suspected.

Tubulovesicular elements in Blastocystis hominis from the caecum of experimentally-infected rats

By transmission electron microscopy (TEM), tubulovesicular elements were seen in Blastocystis hominis obtained from the caecum of experimentally-infected rats. These appeared to arise from the peripheral cytoplasm and were rounded, oval or elongate in sections. It is suggested that these elements form a network for transfer of nutrients to the periphery during the process of encystation.

The flagellar pocket of trypanosomatids

The surface of the trypanosomatid forms the interface between the parasite and its host, and has evolved to repel a variety of host anti-microbial defences. The flagellar pocket constitutes a highly differentiated region of the trypanosomatid surface that facilitates internalization of host macromolecules, while restricting host access to the exposed, endocytic receptors of the parasite. In this review, Paul Webster and David Russell discuss the ability of this organelle to accumulate efficiently nutrients obtained from the host as a major factor in the success of this group of parasites.

5'nucleotidase is sorted to the apical domain of hepatocytes via an indirect route

In hepatocytes, all newly synthesized plasma membrane (PM) proteins so far studied arrive first at the basolateral domain; apically destined proteins are subsequently endocytosed and sorted to the apical domain via transcytosis. A mechanism for the sorting of newly synthesized glycophosphatidylinositol (GPI)-linked proteins has been proposed whereby they associate in lipid microdomains in the trans-Golgi network and then arrive at the apical domain directly. Such a mechanism poses a potential exception to the hepatocyte rule. We have used pulse-chase techniques in conjunction with subcellular fractionation to compare the trafficking of 5' nucleotidase (5NT), an endogenous GPI-anchored protein of hepatocytes, with two transmembrane proteins. Using a one-step fractionation technique to separate a highly enriched fraction of Golgi-derived membranes from ER and PM, we find that both 5NT and the polymeric IgA receptor (pIgAR) traverse the ER and Golgi apparatus with high efficiency. Using a method that resolves PM vesicles derived from the apical and basolateral domains, we find that 5NT first appears at the basolateral domain as early as 30 min of chase. However the subsequent redistribution to the apical domain requires > 3.5 h of chase to reach steady state. This rate of transcytosis is much slower than that observed for dipeptidylpeptidase IV, an apical protein anchored via a single transmembrane domain. We propose that the slow rate of transcytosis is related to the fact that GPI-linked proteins are excluded from clathrin-coated pits/vesicles, and instead must be endocytosed via a slower nonclathrin pathway.

Identification of two distinct galactosyltransferase activities acting on the variant surface glycoprotein of Trypanosoma brucei

Variant surface glycoproteins (VSGs) of Trypanosoma brucei contain two distinct glycosylation sites: (1) N-linked glycans within the protein portion of the molecules, and (2) the glycosyl-phosphatidylinositol (GPI) membrane anchor. Since galactose residues show uncommon alpha-glycosidic linkages in the GPI membrane anchor, we were prompted to investigate galactosylation of the GPI anchor. On comparing a trypanosome clone galactosylated exclusively in N-glycans (clone MITat 1.5) with clones galactosylated predominantly in the glypiated membrane anchor (clones MITat 1.4, MITat 1.6 and AnTat 1.8), clone MITat 1.5 showed a 10-fold increased enzyme activity when using a protocol including Triton X-100 to assay UDPgalactose:N-acetylglucosaminyl glycopeptide beta 1,4-galactosyltransferase (EC 2.4.1.38). Only the VSG of clone MITat 1.5 could be radiochemically labelled with UDP[14C]galactose, and galactosylation of N-glycans was confirmed by digestion with peptide-N4-(N-acetylglucosaminyl)asparagine amidase (PNGase F). However, in a modified enzyme assay without detergent, galactosyltransferase activity was increased considerably (15-fold) in clone MITat 1.4. VSG galactosylation of clones MITat 1.4, MITat 1.6 and AnTat 1.8 was readily detected by fluorography of the respective SDS/polyacrylamide gels, suggesting that galactosyltransferase activity modifies the VSG membrane anchor in these clones. In this case, [14C]galactose labelling of immunoprecipitated VSG (clone MITat 1.4) was resistant to the release of N-glycans by PNGase F treatment, and thus revealed galactosylation in vitro of a VSG membrane anchor. Exoglycosidase digestions of VSG MITat 1.4 confirmed the presence of alpha-linked galactose residues. We suggest that these specific alpha-galactosyltransferases are inhibited by the action of detergent, but can be activated in a detergent-free buffer system.

Surface coat synthesis and turnover from epimastigote to bloodstream forms of Trypanosoma brucei

Monoclonal antibodies to metacyclic surface coat glycoproteins of Trypanosoma brucei brucei STIB 247LG were produced for a study of the synthesis of metacyclic variable surface glycoproteins (VSGs) within the salivary gland of Glossina morsitans morsitans, and of the first exchange of the surface glycoproteins after infection in mice. Immunofluorescence antibody tests and protein A-gold labelling revealed that the VSGs are continuously integrated into the whole surface of the trypanosome while it is still attached to the gland epithelium. A pool of 8 antibodies recognized about 50% of the metacyclic forms present in the saliva of an infected tsetse fly, which confirmed the heterogeneity of the metacyclic VSG-generation. The labelling experiments showed that the integration of the first VSG-generation into the surface of bloodstream forms takes place in the same way as in the metacyclics. This process started on day 3 after infection and was finished on day 6.

Degradation, recycling, and shedding of Trypanosoma brucei variant surface glycoprotein

Trypanosoma brucei bloodstream forms express a densely packed surface coat consisting of identical variant surface glycoprotein (VSG) molecules. This surface coat is subject to antigenic variation by sequential expression of different VSG genes and thus enables the cells to escape the mammalian host's specific immune response. VSG turnover was investigated and compared with the antigen switching rate. Living cells were radiochemically labeled with either 125I-Bolton-Hunter reagent or 35S-methionine, and immunogold-surface labeled for electron microscopy studies. The fate of labeled VSG was studied during subsequent incubation or cultivation of labeled trypanosomes. Our data show that living cells slowly released VSG into the medium with a shedding rate of 2.2 +/- 0.6% h-1 (t1/2 = 33 +/- 9 h). In contrast, VSG degradation accounted for only 0.3 +/- 0.06% h-1 (t1/2 = 237 +/- 45 h) and followed the classical lysosomal pathway as judged by electron microscopy. Since VSG uptake by endocytosis was rather high, our data suggest that most of the endocytosed VSG was recycled to the surface membrane. These results indicate that shedding of VSG at a regular turnover rate is sufficient to remove the old VSG coat within one week, and no increase of the VSG turnover rate seems to be necessary during antigenic variation.

Expression site associated genes of Trypanosoma brucei rhodesiense

Upstream of at least some telomere-linked genes for the variant surface glycoproteins (VSGs) of African trypanosomes are expression site associated genes (ESAGs) whose transcription is co-ordinated with the transcription of the adjacent VSG gene [Cully et al. (1985) Cell 42, 173-182]. The function of the corresponding ESAG proteins is not known. Here we show the sequences of two members of the ESAG-I family that are upstream of the VSG genes expressed in metacyclic variant antigen types 4 and 7 of Trypanosoma brucei rhodesiense. The corresponding metacyclic ESAG-I proteins of about 330 amino acids display extensive positional identity both with each other and with two other ESAG-I proteins of Trypanosoma brucei brucei. Only about 7% of the positions are occupied by a different amino acid in each of the four putative ESAG proteins while 40% of the positions are identical. Thus, the ESAG-I proteins are much more highly conserved than are the VSGs studied to date.