In situ architecture of the ciliary base reveals the stepwise assembly of IFT trains

Abstract

The cilium is an antenna-like organelle that performs numerous cellular functions, including motility, sensing, and signaling. The base of the cilium contains a selective barrier that regulates the entry of large intraflagellar transport (IFT) trains, which carry cargo proteins required for ciliary assembly and maintenance. However, the native architecture of the ciliary base and the process of IFT train assembly remain unresolved. Here, we use in situ cryo-electron tomography to reveal native structures of the transition zone region and assembling IFT trains at the ciliary base. We combine this direct cellular visualization with ultrastructure expansion microscopy to describe the front-to-back stepwise assembly of IFT trains: IFT-B forms the backbone, onto which IFT-A, then dynein-1b, and finally kinesin-2 sequentially bind before entry into the cilium.

Restriction of intraflagellar transport to some microtubule doublets: An opportunity for cilia diversification?

“…Very recently, a combination of cryo‐electron microscopy and expansion microscopy data revealed that IFT trains are assembled around the basal body in Chlamydomonas . [ 43 ] These trains could form exclusively along some triplets of the basal body, according to the associated structural and molecular features mentioned above. Once the kinesin motor gets in action, its processivity would ensure that the trains travel on the doublet where it was assembled.…”

Integrative modeling reveals the molecular architecture of the Intraflagellar Transport A (IFT-A) complex

“…While crystal structures have been determined for several IFT-B subunits (29–34), no such structures exist for any IFT-A proteins. Recently, cryo-electron tomography (cryo-ET) has provided a valuable in situ view of the assembly of IFT complexes into polymeric structures, known as IFT trains, at the ciliary base, highlighting their stepwise association (35), and capturing snapshots of IFT trains moving along the axonemes (36). These cryo-ET studies have not only revealed the low-resolution structures of the IFT-A and IFT-B complexes, but also revealed their cellular context, the mode of IFT-A/IFT-B associations, and the stoichiometries between the complexes.…”