Assembly mechanism of Trypanosoma brucei BILBO1 at the flagellar pocket collar

Abstract

The flagellar pocket is a bulb-like invagination of the plasma membrane that encloses the base of the single flagellum in trypanosomes. It is the site of all endo- and exocytic activity in the parasite and has thus been proposed to be a therapeutic target. At the neck of the flagellar pocket is an electron-dense cytoskeletal structure named the flagellar pocket collar. The protein BILBO1 was the first characterized and remains the only known component of the flagellar pocket collar, with essential functions in the biogenesis of both the flagellar pocket and flagellar pocket collar. We recently reported that the filamentous assembly of Trypanosoma brucei BILBO1 (TbBILBO1) is mediated by its central coiled coil domain and C-terminal leucine zipper. Here, we discuss how TbBILBO1 might assemble at the flagellar pocket collar in T. brucei.

Keywords: BILBO1; Trypanosoma brucei; cytoskeleton; flagellar pocket; flagellar pocket collar; parasite; protein assembly.

Figure 1.

Schematic of the structure and function of the flagellar pocket in trypanosomes. The FP is a bulb-like invagination of the plasma membrane at the base of the flagellum at the cell posterior. Although it represents only 5% of the total cell surface, the FP is responsible for all endo- and exocytosis in the parasite. At the neck of the FP is a belt-like structure called the flagellar pocket collar (FPC). The plasma membrane is shown in black; membranes of the FP and the flagellum are shown in light and dark gray, respectively.

Figure 2.

Putative assembly mechanism of TbBILBO1 at the flagellar pocket collar. (A) Schematic depicting the arrangement of the 4 structural domains of TbBILBO1: N-terminal domain, EF hand motifs, coiled-coil domain, and leucine zipper. (B) A hypothetical model showing the possible assembly of TbBILBO1 at the FPC. Two monomeric TbBILBO1 molecules form an antiparallel dimer via their central coiled-coil domains, which further assemble into a long filamentous oligomer through the intermolecular interactions of the C-terminal leucine zippers. (C) A hypothetical model showing that the filament forms a solenoid in the FPC to encircle the flagellar pocket neck. An enlarged view of the boxed region shows the possible lateral interactions mediated by the EF-hand motifs (dashed arrows). The N-terminal domain might interact with an unknown FPC component or a regulatory protein (red arrows and hexagons) to facilitate FPC assembly. (D) An alternative model showing a horseshoe-like arrangement of TbBILBO1 filaments at the FPC with the microtubule quartet passing through the gap. A putative capping protein at the tip of the filaments is indicated by orange crescents. MtQ, microtubule quartet.