Existence of cellular structures with specific size raises a fundamental question in biology: How do cells measure length? One conceptual answer to this question is by a molecular ruler, but examples of such rulers in eukaryotes are lacking. In this work, we identified a molecular ruler in eukaryotic cilia and flagella. Using cryo-electron tomography, we found that FAP59 and FAP172 form a 96-nanometer (nm)-long complex in Chlamydomonas flagella and that the absence of the complex disrupted 96-nm repeats of axonemes. Furthermore, lengthening of the FAP59/172 complex by domain duplication resulted in extension of the repeats up to 128 nm, as well as duplication of specific axonemal components. Thus, the FAP59/172 complex is the molecular ruler that determines the 96-nm repeat length and arrangements of components in cilia and flagella.

The question of what proteins compose the junctions between two tubules in doublet microtubules is long-standing. Here a conserved flagellar protein, FAP20, is shown to be an inner junction component important for stabilizing the axoneme and scaffolding intra–B-tubular structures required for a planar asymmetrical waveform.

Nonspecific intermolecular collision between the central pair apparatus and radial spokes underlies a mechanosensing mechanism that regulates dynein activity in Chlamydomonas flagella.

KIF1A is a kinesin family motor involved in the axonal transport of synaptic vesicle precursors (SVPs) along microtubules (MTs). In humans, more than 10 point mutations in KIF1A are associated with the motor neuron disease hereditary spastic paraplegia (SPG). However, not all of these mutations appear to inhibit the motility of the KIF1A motor, and thus a cogent molecular explanation for how KIF1A mutations lead to neuropathy is not available. In this study, we established in vitro motility assays with purified full-length human KIF1A and found that KIF1A mutations associated with the hereditary SPG lead to hyperactivation of KIF1A motility. Introduction of the corresponding mutations into the Caenorhabditis elegans KIF1A homolog unc-104 revealed abnormal accumulation of SVPs at the tips of axons and increased anterograde axonal transport of SVPs. Our data reveal that hyperactivation of kinesin motor activity, rather than its loss of function, is a cause of motor neuron disease in humans.axonal transport | hereditary spastic paraplegia | kinesin | KIF1A | UNC-104

Poliovirus (PV), when injected intramuscularly into the calf, is incorporated into the sciatic nerve and causes an initial paralysis of the inoculated limb in transgenic mice carrying the human PV receptor (hPVR/CD155) gene. Here, we demonstrated by using an immunoelectron microscope that PV particles exist on vesicle structures in nerve terminals of neuromuscular junctions. We also demonstrated in glutathione S-transferase pulldown experiments that the dynein light chain, Tctex-1, interacts directly with the cytoplasmic domain of hPVR. In the axons of differentiated rat PC12 cells transfected with expression vectors for hPVRs, vesicles composed of PV and hPVR␣, as well as a mutant hPVR␣ (hPVRM␣) that had a reduced ability to bind Tctex-1, colocalized with Tctex-1. However, vesicles containing PV, dextran, and hPVR␣ had only retrograde motion, while those containing PV, dextran, and hPVRM␣ had anterograde or retrograde motion. Topical application of the antimicrotubule agent vinblastine to the sciatic nerve reduced the amount of virus transported from the calf to the spinal cord. These results suggest that direct efficient interaction between the cytoplasmic domain and Tctex-1 is essential for the efficient retrograde transport of PV-containing vesicles along microtubules in vivo.In humans, paralytic poliomyelitis is considered to result from an invasion by circulating poliovirus (PV) into the central nervous system, probably via the blood-brain barrier. The notion is supported by a previous study using a mouse model (44). In that study, it was demonstrated that circulating PV after intravenous inoculation appears to cross the blood-brain barrier at a high rate, and the neural dissemination pathway from the skeletal muscle is not the primary dissemination route of the circulating virus to the central nervous system. Along with this pathway of dissemination, a neural pathway has been reported in humans (32), monkeys (20), and PV-sensitive transgenic (Tg) mice carrying the human PV receptor (hPVR/ CD155) gene (35,36), and it appears to be important in causing provocation poliomyelitis (14). Using the Tg mouse line, we demonstrate that PV inoculated into the calf is incorporated into the sciatic nerve and retrogradely transported through the axons as intact virion particles, that one of the fast retrograde axonal transport systems is involved in viral dissemination, and that the pathogenesis of PV infection via the neural pathway is inhibited by the anti-hPVR monoclonal antibody (MAb) p286, which is able to block the infection (35).hPVR is a member of the immunoglobulin (Ig) superfamily, with three linked extracellular Ig-like domains followed by a membrane-spanning domain and a cytoplasmic domain (CP). Two membrane-bound forms (hPVR␣ and hPVR␦) and two secreted forms (hPVR␤ and hPVR␥) derived by alternative splicing are potentially expressed in human cells (28). Membrane-bound hPVRs are considered to play important roles in the early steps of infection, such as binding of the virus to the cell surface, penetration...