Intraflagellar Transport - Biochemistry

Biochemistry

IFT describes the bi-directional movement of non-membrane-bound particles along the doublet microtubules of the flagellar axoneme, between the axoneme and the plasma membrane. Studies have shown that the movement of IFT particles along the microtubule is carried out by two different microtubule-based motors; the anterograde (towards the flagellar tip) motor is heterotrimeric kinesin-2, and the retrograde (towards the cell body) motor is cytoplasmic dynein 1b. IFT particles carry axonemal subunits to the site of assembly at the tip of the axoneme; thus, IFT is necessary for axonemal growth. Therefore, since the axoneme needs a continually fresh supply of proteins, an axoneme with defective IFT machinery will slowly shrink in the absence of replacement protein subunits. In healthy flagella, IFT particles reverse direction at the tip of the axoneme, and are thought to carry used proteins, or "turnover products," back to the base of the flagellum.

The IFT particles themselves consist of two sub-complexes, each made up of several individual IFT proteins. The two complexes, known as 'A' and 'B,' are separable via sucrose centrifugation (both complexes at approximately 16S, but under increased ionic strength complex B sediments more slowly, thus segregating the two complexes). The many subunits of the IFT complexes have been named according to their molecular weights:

• complex A contains IFT144, IFT140, IFT139, and IFT122
• complex B contains IFT172, IFT88, IFT81, IFT80, IFT74, IFT57, IFT52, IFT46, IFT27, and IFT20

The biochemical properties and biological functions of these IFT subunits are just beginning to be elucidated, for example they interact with components of the basal body like Cep170 or proteins which are required for cilium formation like tubulin chaperone and membrane proteins.