Localized mRNA translation mediates maturation of cytoplasmic cilia in Drosophila spermatogenesis

Abstract

32Cytoplasmic cilia, a specialized type of cilia in which the axoneme resides within the 33 cytoplasm rather than within the ciliary compartment, are proposed to allow the efficient 34 assembly of very long cilia. Despite being found diversely in male gametes (e.g. Plasmodium 35microgametocytes and human and Drosophila sperm), very little is known about cytoplasmic 36 cilia assembly. Here we show that a novel RNP granule containing the mRNAs for axonemal 37 dynein motor proteins becomes highly polarized to the distal end of the cilia during 38 cytoplasmic ciliogenesis in Drosophila sperm. This allows for the localized translation of 39 these axonemal dyneins and their incorporation into the axoneme directly from the 40 cytoplasm. We found that this RNP granule contains the proteins Reptin and Pontin, loss of 41 which perturbs granule formation and prevents incorporation of the axonemal dyneins, 42 leading to sterility. We propose that cytoplasmic cilia require the local translation of key 43 protein constituents such that these proteins are incorporated efficiently into the axoneme. 44 45 Author Summary 46Cytoplasmic cilia, which are found in human and Drosophila sperm, are unique in that 47 the axoneme is exposed to the cytoplasm. The authors show that a novel RNP granule 48 containing axonemal dynein mRNAs facilitates localized translation of these axonemal 49 proteins, facilitating cytoplasmic cilia formation. 50 51 Abbreviations 52 SC spermatocyte 53 IFT intraflagellar transport 54 ODA Outer dynein arm 55 IDA Inner dynein arm 56 RNP Ribonucleoprotein 57 smFISH single molecule RNA fluorescent in situ hybridization 58 IF Immunofluorescent 59 IC Individualization complex 60 TEM Transmission light microscopy 61 62

Functional partitioning of a liquid-like organelle during assembly of axonemal dyneins

“…First, a recent paper reports that splicing factor Srsf1 plays a splicing-independent role in ciliary beating, likely via control of translation of protein essential for ciliary beating ( Haward et al, 2020 ). A second recent paper reports the presence of ‘KL granules’ in Drosophila sperm that -like DynAPs- contain the Dynein assembly chaperones Ruvbl1/2, and these granules are also highly enriched in the mRNAs encoding axonemal dyneins ( Fingerhut and Yamashita, 2020 ). Taken together, these findings suggest that the role of RNA-associated proteins in DynAP assembly or function should be a rich area for future investigation.…”