Chlamydomonas: Intraflagellar Transport

Cilia/flagella are microtubule-based organelles emanating from the surface of most eukaryotic cells and accomplish motile and sensory functions. Malfunction of cilia results in a variety of ciliopathies such as polycystic kidney disease, retinal degeneration, and male infertility. The assembly of nearly all cilia/flagella depends on a conserved transport machinery–intraflagellar transport (IFT), which has 22 subunits and is composed of IFT-A and IFT-B complex. IFT moves bidirectionally in flagella and is driven by the anterograde motor kinesin-2 and the retrograde motor dynein 1b/2. Co-expression and crystal structure analysis of IFT complex demonstrated that IFT-B can be divided into IFT-B1 and IFT-B2 subcomplex, and each subcomplex has a tubulin-binding module, suggesting that tubulin is a primary cargo of IFT. Other cargoes include axonemal components from outer dynein arms (ODA), inner dynein arms (IDA), radial spoke proteins (RSP) and dynein regulatory complexes (DRC), and membrane proteins. The IFT and cargoes form long trains and short trains in flagella under the electron microscopy. By transporting the flagellar precursors, turnover products, or signal molecules, IFT plays a critical role in flagellar assembly, disassembly, and signal transduction in Chlamydomonas. In addition to moving in flagella, IFT proteins also locate at the basal body and tip of flagella, where the regulation of IFT occurs. Future studies need to illustrate the interaction mechanism between the IFT and cargoes, relationship between IFT and the flagellar membrane trafficking, and the complexity and flexibility of the structure of IFT in vivo and to reconstitute the IFT machinery in vitro.