Sorting Signals Required for Trafficking of the Cysteine-Rich Acidic Repetitive Transmembrane Protein in Trypanosoma brucei

Abstract

The African trypanosome Trypanosoma brucei is a unicellular eukaryotic parasite, causing sleeping sickness in humans and related diseases in livestock. T. brucei has an intricate life cycle, alternating between the bloodstream form, which is free living in the bloodstream of a mammalian host, and the procyclic (or insect) form, which lives in the transmitting vector, the tsetse fly. By undergoing antigenic variation of the variant surface glycoprotein (VSG) coat, which covers the entire cell surface of the bloodstream form, the parasite survives the host immune attack (10, 16). During differentiation into the procyclic form in the midgut of the tsetse fly, the parasite sheds its VSG coat and replaces it with the procyclin (or the procyclic acidic repetitive protein) coat (42).T. brucei, an extracellular organism, is dependent on hostderived nutrients for its growth and development. Underneath a large part of the plasma membrane is the subpellicular microtubule sheath. This distinct surface structure network prohibits the active pinocytosis of plasma membrane as a means for nutrient uptake. The only surface domain where the microtubule network is absent is the flagellar pocket (FP); the FP is an invagination of the plasma membrane, where the flagellum extends from the cell (4,49,50,53). In trypanosomatids, endocytosis and exocytosis are restricted to the FP. Receptors for the uptake of macromolecules are confined to the FP (39, 53). All vesicular trafficking in this elongated and highly polarized parasite takes place between the nucleus and the FP (11,19,31,39,40,53). Membrane-bound proteins, once synthesized, travel from the endoplasmic reticulum (ER) to the Golgi/trans-Golgi network (TGN) and then to the FP membrane. From the pocket, surface coat proteins and some other invariant surface proteins rapidly spread over the entire cell surface, while receptors for the uptake of macromolecules are retained in the FP. We have been investigating mechanisms involved in the sorting of membrane proteins to and from the FP in trypanosomes, a phenomenon that is poorly understood. In the past, most studies on protein trafficking in trypanosomes have focused on the bloodstream form, due to its unique high rate of endocytosis (2,14,17,33,34,51,52). Endocytosis at the FP of the bloodstream form is mediated by large clathrincoated vesicles (2,(18)(19)(20)32). These vesicles are undetectable in the procyclic form, where endocytosis occurs in a much less efficient manner than in the bloodstream form (26).Little is known about the molecular details involved in protein sorting during endocytosis and along the secretory pathway in trypanosomes, though trafficking machinery similar to that of higher eukaryotes may operate (33,34,40). Based on studies in mammalian systems, a general principle underlying directional transfer is that the sorting information (sorting signals or determinants), usually distinct amino acid sequences or structural domains, resides within the sorted protein. The interaction of sorting determinants with specif...

Multiple Motifs Regulate Trafficking of the LAMP‐Like Protein p67 in the Ancient Eukaryote Trypanosoma brucei

“…These studies did present contradictory results on the effect of CHC ablation on p67 localization, but CRAM localization was clearly disrupted (36). Consistent with the later finding, the CRAM CD was found in yeast two‐hybrid assays to interact in a sequence‐specific manner with the μ‐1 subunit of the trypanosomal AP‐1 complex (32).…”

“…In the type I transmembrane flagellar pocket protein CRAM, overlapping motifs in the CD mediate both ER‐exit and flagellar pocket retention (31). Although there are no dileucine or tyrosine motifs in the CRAM CD, mutagenesis of specific amino acids did impair endocytosis (32).…”

Intracellular Trafficking in the Trypanosomatids

“…In an interesting study, Qiao et al investigated the trafficking of CRAM a type I transmembrane highly repetitive protein that localises to the flagellar pocket, and which may be a lipoprotein receptor (72). Using an alanine insertion strategy for the cytoplasmic domain, these authors demonstrated effects on both ER exit and endocytosis; disruption mapped to distinct regions of the cytoplasmic tail for exocytosis and endocytosis.…”

Leishmania Adaptor Protein-1 Subunits Are Required for Normal Lysosome Traffic, Flagellum Biogenesis, Lipid Homeostasis, and Adaptation to Temperatures Encountered in the Mammalian Host

“…RNA interference knockdown of AP-1 subunits leads to pleiotropic effects in endosomal membrane traffic and cell division, although protein transport to the lysosome is apparently not affected (1,44). In contrast, genetic deletion of AP-1 1 or 1 subunits in Leishmania mexicana mexicana has little effect on the growth of promastigote stages in rich medium (19).…”