Abstract
ABSTRACTPrimary cilia are compartmentalised from the rest of the cell by a ciliary gate comprising transition fibres and a transition zone. The ciliary gate allows the selective import and export of molecules such as transmembrane receptors and transport proteins. These are required for the assembly of the cilium, its function as a sensory and signalling centre and to maintain its distinctive composition. Certain motile cilia can also form within the cytosol as exemplified by human and Drosophila sperm. The role of transition fibre proteins has not been well described in the cytoplasmic cilia.Drosophila have both compartmentalised primary cilia, in sensory neurons, and sperm flagella that form within the cytosol. Here, we describe phenotypes for twitchy the Drosophila orthologue of a transition fibre protein, mammalian FBF1/C. elegans dyf-19. Loss-of-function mutants in twitchy are adult lethal and display a severely uncoordinated phenotype. Twitchy flies are too uncoordinated to mate but RNAi-mediated loss of twitchy specifically within the male germline results in coordinated but infertile adults. Examination of sperm from twitchy RNAi-knockdown flies shows that the flagellar axoneme forms, elongates and is post-translationally modified by polyglycylation but the production of motile sperm is impaired. These results indicate that twitchy is required for the function of both sensory cilia that are compartmentalised from the rest of the cell and sperm flagella that are formed within the cytosol of the cell. Twitchy is therefore likely to function as part of a molecular gate in sensory neurons but may have a distinct function in sperm cells.
Highlights
Primary cilia are hair-like microtubule based projections that extend from the cell
Twitchy mutant flies are uncoordinated From a screen for lethal mutations within the cytological region 68D of the Drosophila genome, we identified a class of loss-of-function mutants that gave rise to an uncoordinated phenotype
Twitchy is an evolutionary conserved protein required for specialised ciliary function We have identified mutations in the Drosophila orthologue of human FBF1/C. elegans dyf-19, the first mutations reported in flies for a molecular component of the transition fibres
Summary
Primary cilia are hair-like microtubule based projections that extend from the cell (reviewed in Ishikawa and Marshall, 2011). Initial extension of microtubule doublets from the basal body produces a specialised region, the transition zone In primary cilia this forms part of a ‘ciliary gate’ that compartmentalises the cilium from the rest of the cell and regulates the movement of large molecules (reviewed in Garcia-Gonzalo and Reiter, 2017; Reiter et al, 2012). Proteomic (Tanos et al, 2013) and super resolution microscopy studies (Bowler et al, 2019; Yang et al, 2018) in mammalian cells have identified several molecular components of the distal appendages/transition fibres and the structures they form at the base of the cilia In these structures FBF1 localises to a matrix (Yang et al, 2018) or outer ring (Bowler et al, 2019) facilitating its interactions with molecules in the vicinity of the basal body. The C. elegans FBF1 orthologue dyf-19, localises to the ciliary base and facilitates the ciliary import of assembled IFT particles (Wei et al, 2013)
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