Abstract
ABSTRACTEukaryotic flagella are complex microtubule-based organelles that, in many organisms, contain extra-axonemal structures, such as the outer dense fibres of mammalian sperm and the paraflagellar rod (PFR) of trypanosomes. Flagellum assembly is a complex process occurring across three main compartments, the cytoplasm, the transition zone and the flagellum itself. The process begins with the translation of protein components followed by their sorting and trafficking into the flagellum, transport to the assembly site and incorporation. Flagella are formed from over 500 proteins and the principles governing assembly of the axonemal components are relatively clear. However, the coordination and location of assembly of extra-axonemal structures are less clear. We have discovered two cytoplasmic proteins in Trypanosoma brucei that are required for PFR formation, PFR assembly factors 1 and 2 (PFR-AF1 and PFR-AF2, respectively). Deletion of either PFR-AF1 or PFR-AF2 dramatically disrupted PFR formation and caused a reduction in the amount of major PFR proteins. The existence of cytoplasmic factors required for PFR formation aligns with the concept that processes facilitating axoneme assembly occur across multiple compartments, and this is likely a common theme for extra-axonemal structure assembly.
Highlights
The eukaryotic flagellum is a well conserved organelle with multiple functions, which include providing a propulsive force and acting as a sensory platform (Moran et al, 2014)
Some components of the axoneme seem to travel to the flagellum tip as individual proteins, others are pre-assembled in the cytoplasm; all are influenced by the three main compartments required for flagellum assembly
We have shown that two proteins, paraflagellar rod (PFR)-AF1 and PFR assembly factor 2 (PFR-AF2), form a cytoplasmic complex important for PFR assembly
Summary
The eukaryotic flagellum is a well conserved organelle with multiple functions, which include providing a propulsive force and acting as a sensory platform (Moran et al, 2014). In addition to the dynein proteins themselves, disruption of other proteins can cause the loss of the axonemal outer and inner dynein arms, resulting in flagellar motility defects. Investigation of these proteins, predominantly in Chlamydomonas, has led to the discovery of an ordered axonemal dynein assembly process that has three key steps each occurring in each of the three main compartments: (1) cytoplasmic assembly and maturation of the outer dynein arm complex, (2) transport of the complex into the flagellum and (3) docking of the complex to the microtubule doublet (Desai et al, 2018). Some components of the axoneme seem to travel to the flagellum tip as individual proteins, others are pre-assembled in the cytoplasm; all are influenced by the three main compartments required for flagellum assembly
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