Mechanism of IFT-A polymerization into trains for ciliary transport

Intraflagellar transport (IFT) is the highly conserved process by which proteins are transported along ciliary microtubules by a train-like polymeric assembly of IFT-A and IFT-B complexes. IFT-A is sandwiched between IFT-B and the ciliary membrane, consistent with its putative role in transporting transmembrane and membrane-associated cargoes. Here, we have used single-particle analysis electron cryomicroscopy (cryo-EM) to determine structures of native IFT-A complexes. We show that subcomplex rearrangements enable IFT-A to polymerize laterally on anterograde IFT trains, revealing a cooperative assembly mechanism. Surprisingly, we discover that binding of IFT-A to IFT-B shields the preferred lipid-binding interface from the ciliary membrane but orients an interconnected network of b-propeller domains with the capacity to accommodate diverse cargoes toward the ciliary membrane. This work provides a mechanistic basis for understanding IFT-train assembly and cargo interactions.

Automated summary

This study determined the structures of native IFT-A complexes using cryo-EM and found that subcomplex rearrangements enable IFT-A to polymerize on anterograde IFT trains. The study also discovered that binding of IFT-A to IFT-B shields the preferred lipid-binding interface and orients a network of b-propeller domains towards the ciliary membrane, capable of accommodating diverse cargoes.