FAP206 is a microtubule-docking adapter for ciliary radial spoke 2 and dynein c.
Bottom Line: A knockout of the conserved ciliary protein FAP206 in the ciliate Tetrahymena resulted in slow cell motility.Cryo-electron tomography showed that in the absence of FAP206, the 96-nm repeats lacked RS2 and dynein c.Overexpressed GFP-FAP206 decorated nonciliary microtubules in vivo.
Affiliation: Department of Cellular Biology, University of Georgia, Athens, GA 30602.Show MeSH
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Mentions: A detailed comparison of the subtomogram averages of all repeats of the FAP206-KO and wild-type axonemes revealed that in FAP206-KO, the electron density of RS1 is unaffected, RS2 is greatly reduced, but a faint signal is still detectable, and RS3 is mildly reduced (Figures 3, A and B, and 4, A and B). A reduced electron density in specific areas of a subtomogram average is an indication that the individual repeats are not identical. Heterogeneity among the individual 96-nm repeats could be either caused by flexibility (i.e., the position of a structure varies between the individual repeats) or because a structure is absent in a subset of repeats. We used automatic image classification (Heumann et al., 2011) to sort the axonemal repeats into homogeneous subgroups before generating class averages. For wild type, we could not identify classes that differed in the structures of radial spokes (Figures 3A and 4A). When we classified for differences in the RS2 structure among the FAP206-KO subtomograms, the majority of axonemal repeats (83%) lacked RS2 entirely (except for the side prong; Figure 3, D and F), whereas the remaining 17% (FAP206-KO [+RS2] class) had a nearly completely assembled RS2 but lacked parts of the microtubule base (Figure 3, C and E). A comparison of the wild-type RS2 structure with the FAP206-KO (+RS2) class revealed a fairly normal stem and head of RS2, whereas the entire front prong and the closely associated dynein c were missing, and the back prong was reduced (compare Figures 2G and 3C, and 2C and 3E; also see Supplemental Movie S7). We estimated the molecular weight of the front prong density that is missing in the FAP206-KO (+RS2) class average as ∼80 kDa (red region in Figure 2, C, D, and K), which is consistent with the predicted (73 kDa) and observed molecular weight (80 kDa; see later discussion of Figure 7D) of FAP206. The simplest explanation of these observations is that FAP206 forms the front prong of RS2.
Affiliation: Department of Cellular Biology, University of Georgia, Athens, GA 30602.