The nexin–dynein regulatory complex (N-DRC) in motile cilia and flagella functions as a linker between neighboring doublet microtubules, acts to stabilize the axonemal core structure, and serves as a central hub for the regulation of ciliary motility. Although the N-DRC has been studied extensively using genetic, biochemical, and structural approaches, the precise arrangement of the 11 (or more) N-DRC subunits remains unknown. Here, using cryo-electron tomography, we have compared the structure of Chlamydomonas wild-type flagella to that of strains with specific DRC subunit deletions or rescued strains with tagged DRC subunits. Our results show that DRC7 is a central linker subunit that helps connect the N-DRC to the outer dynein arms. DRC11 is required for the assembly of DRC8, and DRC8/11 form a subcomplex in the proximal lobe of the linker domain that is required to form stable contacts to the neighboring B-tubule. Gold labeling of tagged subunits determines the precise locations of the previously ambiguous N terminus of DRC4 and C terminus of DRC5. DRC4 is now shown to contribute to the core scaffold of the N-DRC. Our results reveal the overall architecture of N-DRC, with the 3 subunits DRC1/2/4 forming a core complex that serves as the scaffold for the assembly of the “functional subunits,” namely DRC3/5–8/11. These findings shed light on N-DRC assembly and its role in regulating flagellar beating.

Background:Integrins undergo large conformational changes when ligand-bound. Results: Using nanodisc technology and EM, we observed Mn 2ϩ -activated ␣ IIb ␤ 3 integrin both alone and fibrin-bound. Conclusion: MnCl 2 -activated ␣ IIb ␤ 3 integrin alone has a compact conformation, becoming fully upright and open when fibrin-bound. Significance: The first structure of membrane-embedded integrins bound to physiological substrate reveals the importance of integrin extension in macromolecular ligand binding.

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