In situ structural analysis of the flagellum attachment zone in Trypanosoma brucei using cryo-scanning transmission electron tomography

The flagellum of Trypanosoma brucei is a 20 µm-long organelle responsible for locomotion and cell morphogenesis. The flagellum attachment zone (FAZ) is a multi-protein complex whose function is to attach the flagellum to the cell body but also to guide cytokinesis. Cryo-transmission electron microscopy is a tool of choice to access the structure of the FAZ in a close-to-native state. However, because of the large dimension of the cell body, the whole FAZ cannot be structurally studied in situ at the nanometre scale in 3D using classical transmission electron microscopy approaches. In the present work, cryo-scanning transmission electron tomography, a new method capable of investigating cryo-fixed thick biological samples, has been used to study whole T. brucei cells at the bloodstream stage. The method has been used to visualise and characterise the structure and organisation of the FAZ filament. It is composed of an array of cytoplasmic stick-like structures. These sticks are heterogeneously distributed between the posterior part and the anterior tip of the cell. This cryo-STET investigation provides new insights into the structure of the FAZ filament. In combination with protein structure predictions, this work proposes a new model for the elongation of the FAZ.