In vivo imaging shows continued association of several IFT-A, IFT-B and dynein complexes while IFT trains U-turn at the tip

Abstract

Flagellar assembly depends on intraflagellar transport (IFT), a bidirectional motility of protein carriers, the IFT trains. The trains are periodic assemblies of IFT-A and IFT-B subcomplexes and the motors kinesin-2 and IFT dynein. At the tip, anterograde trains are remodeled for retrograde IFT, a process that in Chlamydomonas involves kinesin-2 release and train fragmentation. However, the degree of train disassembly at the tip remains unknown. Two-color imaging of fluorescent protein-tagged IFT components indicates that IFT-A and IFT-B proteins from a given anterograde train usually return in the same set of retrograde trains. Similarly, concurrent turnaround was typical for IFT-B proteins and the IFT dynein subunit D1bLIC-GFP but severance was observed as well. Our data support a simple model of IFT turnaround, in which IFT-A, IFT-B, and IFT dynein typically remain associated at the tip and anterograde trains convert directly into retrograde trains without disassembly but for possible splitting into strings of IFT complexes. Continuous association of IFT-A, IFT-B and IFT dynein during tip remodeling could balance protein entry and exit preventing the build-up of IFT material in flagella.

The Molecular Structure of Anterograde Intraflagellar transport trains

“…This indicates that anterograde to retrograde remodelling does not require specialized machineries of the ciliary tip. In Chlamydomonas , the constituents from one anterograde train appear to split into two or three retrograde trains, with IFTA and IFTB complexes remaining associated during anterograde to retrograde conversion 39 . Together, this supports a model in which conversion occurs through conformational changes pre-built into the anterograde train.…”

IFT-A Structure Reveals Carriages for Membrane Protein Transport into Cilia

“…To date, the major reported target of ICK and CDPK1 is kinesin-2 (Chaya et al, 2014; Liang et al, 2014), which is proposed to dissociate from the IFT train upon phosphorylation (Liang et al, 2014). Kinesin-2 dissociation may initiate the major conformational change that IFT trains undergo as they convert to retrograde trains (Chien et al, 2017; Jordan et al, 2018; Stepanek and Pigino, 2016; Wingfield et al, 2021; Zhang et al, 2021).…”

The IFT81-IFT74 complex enhances GTP hydrolysis to inactivate RabL2 during early steps of intraflagellar transport

“…In C. elegans , tracking experiments show that IFT-A and IFT-B components have different dwelling times at the ciliary tip, suggesting that IFT-trains are broken into separate IFT complexes (Mijalkovic et al ., 2018). However, recent work on Chlamydomonas show that IFT-A, IFT-B and IFT dynein subcomplexes stay associated through the switch from anterograde to retrograde IFT at the ciliary tip (Wingfield et al ., 2021).…”