SUMMARY Dyx1c1 has been associated with dyslexia and neuronal migration in the developing neocortex. Unexpectedly, we found that deletion of Dyx1c1 exons 2–4 in mice caused a phenotype resembling primary ciliary dyskinesia (PCD), a genetically heterogeneous disorder characterized by chronic airway disease, laterality defects, and male infertility. This phenotype was confirmed independently in mice with a Dyx1c1c.T2A start codon mutation recovered from an ENU mutagenesis screen. Morpholinos targeting dyx1c1 in zebrafish also created laterality and ciliary motility defects. In humans, recessive loss-of-function DYX1C1 mutations were identified in twelve PCD individuals. Ultrastructural and immunofluorescence analyses of DYX1C1-mutant motile cilia in mice and humans revealed disruptions of outer and inner dynein arms (ODA/IDA). DYX1C1 localizes to the cytoplasm of respiratory epithelial cells, its interactome is enriched for molecular chaperones, and it interacts with the cytoplasmic ODA/IDA assembly factor DNAAF2/KTU. Thus, we propose that DYX1C1 is a newly identified dynein axonemal assembly factor (DNAAF4).

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Abstract. Clear images of myosin filaments have been seen in shadowed freeze-fracture replicas of single fibers of relaxed frog semitendinosus muscles rapidly frozen using a dual propane jet freezing device. These images have been analyzed by optical diffraction and computer averaging and have been modelled to reveal details of the myosin head configuration on the fighthanded, three-stranded helix of cross-bridges. Both the characteristic 430-A and 140-150-A repeats of the myosin cross-bridge array could be seen. The measured filament backbone diameter was 140-160 ~, and the outer diameter of the cross-bridge array was 300 A. Evidence is presented that suggests that the observed images are consistent with a model in which both of the heads of one myosin molecule tilt in the same direction at an angle of -50-70 ° to the normal to the filament long axis and are slewed so that they lie alongside each other and their radially projected density lies along the three right-handed helical tracks. Any perturbation of the myosin heads away from their ideal lattice sites needed to account for x-ray reflections not predicted for a perfect helix must be essentially along the three helical tracks of crossbridges. Little trace of the presence of non-myosin proteins could be seen.horough knowledge of the structure of the myosincontaining filaments in both muscle and non-muscle contractile systems is clearly essential if the mechanism of force production in these systems is to be understood. In particular, x-ray diffraction analysis of cross-bridge movements associated with contraction (9, 10) is likely to require the prior knowledge of the cross-bridge organization on the myosin filaments in relaxed muscle (24).The last few years have seen a significant advance in myosin filament studies. Beautiful electron micrographs of isolated myosin filaments from various invertebrate muscles have now been obtained (14,19,30) and these show for the first time excellent preservation of the helical arrays of myosin crossbridges. In all cases, details of the organization of cross-bridges within the helical arrays have been obtained by optical diffraction and by three-dimensional reconstruction methods. More recently, electron micrographs of much improved preparations of isolated vertebrate muscle myosin filaments have been obtained (13). These confirm the three-stranded helical symmetry of the myosin cross-bridge array (23), and they show that the strands are right-handed. However, apart from the general conclusion that the cross-bridge density lies essentially along these strands, there appears to be little detailed information so far about the organization of myosin heads within the three-stranded helix.Another approach to the study of myosin filament structure is to optimize the preservation of the cross-bridge array in whole muscle fibers by means of rapid freezing methods (1, 2, 7). The ultrastructure in the fibers can then be visualized using freeze-fracturing, deep-etching, heavy metal shadowing, and carbon replication. As was reported...

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