Abstract
Trypanosoma brucei is a uniflagellated protist and the causative agent of African trypanosomiasis, a neglected tropical disease. The single flagellum of T. brucei is essential to a number of cellular processes such as motility, and has been a longstanding focus of scientific enquiry. A number of cytoskeletal structures are associated with the flagellum in T. brucei, and one such structure—a multiprotein complex containing the repeat motif protein TbMORN1—is the focus of this review. The TbMORN1-containing complex, which was discovered less than ten years ago, is essential for the viability of the mammalian-infective form of T. brucei. The complex has an unusual asymmetric morphology, and is coiled around the flagellum to form a hook shape. Proteomic analysis using the proximity-dependent biotin identification (BioID) technique has elucidated a number of its components. Recent work has uncovered a role for TbMORN1 in facilitating protein entry into the cell, thus providing a link between the cytoskeleton and the endomembrane system. This review summarises the extant data on the complex, highlights the outstanding questions for future enquiry, and provides speculation as to its possible role in a size-exclusion mechanism for regulating protein entry. The review additionally clarifies the nomenclature associated with this topic, and proposes the adoption of the term “hook complex” to replace the former name “bilobe” to describe the complex.
Highlights
The trypanosomatids are a group of uniflagellated protists belonging to the class Kinetoplastida in the eukaryotic super-group Excavata [1]
As objects of scientific study, there are at least four reasons why trypanosomatids are of outstanding interest: (i) as pathogens and the causative agents of several neglected tropical diseases, they are responsible for a significant health and economic burden on some of the poorest human communities [2]; (ii) as members of the super-group
The procyclic form (PCF, found in the tsetse fly) and bloodstream forms (BSF, found in mammalian hosts) have been studied the most intensively. Both PCFs and BSFs show trypomastigote morphology, with the single flagellum emerging from the cell posterior and adhering lengthwise to the cell body, with the tip extending beyond the anterior end of the cell [10]
Summary
The trypanosomatids are a group of uniflagellated protists belonging to the class Kinetoplastida in the eukaryotic super-group Excavata [1]. The microtubule quartet is a specialised quartet of microtubules which are presumed to have the opposite polarity to those of the corset [12] They are nucleated between the basal body and probasal body, diagonally wrap around the FP on its cytoplasmic face, and run parallel to the flagellum to the anterior tip of the cell [24]. At the lower end of the flagellar pocket neck, atop the bulge of the FP, is an electron-dense cytoskeletal structure called the flagellar pocket collar (FPC) (Figure 1C) It is horseshoe- or sigma-shaped, with the ends positioned to allow passage of the microtubule quartet [24]. The form (morphology), fabric (composition) and function of the TbMORN1-containing complex will be considered
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