The Flagellum Attachment Zone: 'The Cellular Ruler' of Trypanosome Morphology

Abstract

A defining feature of Trypanosoma brucei cell shape is the lateral attachment of the flagellum to the cell body, mediated by the flagellum attachment zone (FAZ). The FAZ is a complex cytoskeletal structure that connects the flagellum skeleton through two membranes to the cytoskeleton. The FAZ acts as a 'cellular ruler' of morphology by regulating cell length and organelle position and is therefore critical for both cell division and life cycle differentiations. Here we provide an overview of the advances in our understanding of the composition, assembly, and function of the FAZ.

Keywords: FAZ; Trypanosoma brucei; cytoskeleton; flagellum; morphology.

Figure 1

The Morphologies of Kinetoplastid Parasites and the Life Cycle of Trypanosoma brucei. (A) Cartoons showing the common morphologies of kinetoplastid parasites with the anterior and posterior of the cell indicated. The nucleus is the large blue circle and the kinetoplast (mitochondrial DNA) is the small blue oval located at the base of the flagellum. The trypomastigote morphology has the kinetoplast posterior to the nucleus with a long attached flagellum. In the epimastigote and promastigote, the kinetoplast is anterior to the nucleus, but whereas the epimastigote has a long attached flagellum, in the promastigote the flagellum emerges from the anterior pole of the cell. In the amastigote, the kinetoplast is anterior to the nucleus with a small cell body and the flagellum does not emerge beyond the cell body. (B) A simplified cartoon showing the life cycle of T. brucei. The metacyclic form infects the mammalian host where it differentiates first into the slender bloodstream form followed by the stumpy bloodstream form. The fly vector then takes up the stumpy bloodstream form where it differentiates through several stages before differentiating into the mammalian infective metacyclic form in the salivary gland of the fly. For most of its life cycle T. brucei exists as a trypomastigote [indicated by (t)] except for the proventricular epimastigote, the short epimastigote, and the attached epimastigote forms, where it has epimastigote morphology [indicated by (e)].

Figure 2

Organisation of the Flagellum Attachment Zone (FAZ) in Trypanosoma brucei. (A) Simplified cartoon of the key components of a T. brucei trypomastigote cell. Arrows indicate the flagellum, FAZ, flagellar pocket, nucleus, and kinetoplast, with the FAZ highlighted in red and the nucleus and kinetoplast highlighted in blue. The dashed line across the flagellum and FAZ indicates the region where the transverse section in (B) and (C) was taken. (B) Transmission electron microscopy image showing a transverse section of the FAZ with the different zones and structures within them. Abbreviations: ER, endoplasmic reticulum; MTQ, microtubule quartet; PFR, paraflagellar rod. Reproduced from . (C) Cartoon of the transverse section shown in (B) highlighting the asymmetric seam generated in the corset microtubules by the FAZ and MTQ. The five different connections between the various components are shown.

Figure 3

Comparison of the Molecular Organisation of a Desmosome with a Flagellum Attachment Zone (FAZ). (A) Desmosomes are found in multicellular organisms and are large structures that connect together the cytoskeleton of two cells through two plasma membranes; hence, they can be considered analogous to the FAZ. The desmosome model was adapted from and shows the key protein components. The cadherins desmocollin and desmoglein constitute the extracellular core of the desmosome and connect the two halves of the desmosome together. The cadherins are connected into the cytoplasmic plaque by various proteins including plakophilin, plakoglobin, and desmoplakin that in turn bind to the intermediate filaments. (B) Potential interactions and localisations of a selected group of FAZ proteins are shown. Interactions with experimental evidence are shown with a solid line with other possible interactions shown with a dotted line and question mark. Calcium ions are included in the model, as Vickerman noted that Trypanosoma brucei harvested from an animal and then grown in blood containing citrate (a calcium chelator) had detached flagella . This is a highly speculative model; however, it will provide a framework on which to integrate new insights into FAZ protein function and helps to highlight the paucity of knowledge about the FAZ compared with the desmosome.

Figure 4

The Flagellum Attachment Zone (FAZ) Assembly ‘Pull’ Model. (A) A simplified schematic of the Trypanosoma brucei cell cycle. The cell begins to assemble a new flagellum and FAZ (highlighted in red) alongside the old one, followed by kinetoplast division (2). The new flagellum continues to extend alongside the old flagellum until the ‘stop point’ is reached after which the new flagellum does not extend along the old flagellum any further (3). After the stop point is reached the two kinetoplasts separate further, followed by mitosis (4) and finally cytokinesis (5). (B) The FAZ assembly ‘push’ model with the assembling FAZ pushing out the assembled FAZ as the flagellum elongates. FAZ proteins are transported to the proximal site of assembly; however, the exact mechanisms that target these components remain to be elucidated. (C) The FAZ assembly ‘pull’ model with a putative protein complex in the flagellum ‘pulling’ out the assembling FAZ.