Discrete PIH proteins function in the cytoplasmic preassembly of different subsets of axonemal dyneins

Ciliary motility: the components and cytoplasmic preassembly mechanisms of the axonemal dyneins

Motile cilia and flagella are organelles, which function in cell motility and in the transport of fluids over the surface of cells. Motility defects often result in a rare human disease, primary ciliary dyskinesia (PCD). Cell motility depends on axonemal dynein, a molecular motor that drives the beating of cilia and flagella. The dyneins are composed of multiple subunits, which are thought to be preassembled in the cytoplasm before they are transported into cilia and flagella. Axonemal dyneins have been extensively studied in Chlamydomonas. In addition, analyses of human PCDs over the past decade, together with studies in other model animals, have identified the conserved components required for dynein assembly. Recently also, the first cytoplasmic component of dynein assembly, kintoun (ktu), was elucidated through the analysis of a medaka mutant in combination with human genetics and cell biology and biochemical studies of Chlamydomonas. The components of dynein and the proteins involved in its cytoplasmic assembly process are discussed.

A unified taxonomy for ciliary dyneins

The formation and function of eukaryotic cilia/flagella require the action of a large array of dynein microtubule motor complexes. Due to genetic, biochemical, and microscopic tractability, Chlamydomonas reinhardtii has become the premier model system in which to dissect the role of dyneins in flagellar assembly, motility, and signaling. Currently, 54 proteins have been described as components of various Chlamydomonas flagellar dyneins or as factors required for their assembly in the cytoplasm and/or transport into the flagellum; orthologs of nearly all these components are present in other ciliated organisms including humans. For historical reasons, the nomenclature of these diverse dynein components and their corresponding genes, mutant alleles, and orthologs has become extraordinarily confusing. Here, we unify Chlamydomonas dynein gene nomenclature and establish a systematic classification scheme based on structural properties of the encoded proteins. Furthermore, we provide detailed tabulations of the various mutant alleles and protein aliases that have been used and explicitly define the correspondence with orthologous components in other model organisms and humans.

Sperm flagella: comparative and phylogenetic perspectives of protein components

Sperm motility is necessary for the transport of male DNA to eggs in species with both external and internal fertilization. Flagella comprise several proteins for generating and regulating motility. Central cytoskeletal structures called axonemes have been well conserved through evolution. In mammalian sperm flagella, two accessory structures (outer dense fiber and the fibrous sheath) surround the axoneme. The axonemal bend movement is based on the active sliding of axonemal doublet microtubules by the molecular motor dynein, which is divided into outer and inner arm dyneins according to positioning on the doublet microtubule. Outer and inner arm dyneins play different roles in the production and regulation of flagellar motility. Several regulatory mechanisms are known for both dyneins, which are important in motility activation and chemotaxis at fertilization. Although dynein itself has certain properties that contribute to the formation and propagation of flagellar bending, other axonemal structures-specifically, the radial spoke/central pair apparatus-have essential roles in the regulation of flagellar bending. Recent genetic and proteomic studies have explored several new components of axonemes and shed light on the generation and regulation of sperm motility during fertilization.