Assembly and mother centriole recruitment of IFT-B subcomplexes to form IFT-B holocomplex

For the biogenesis and maintenance of cilia, bidirectional protein trafficking within cilia is crucial, and is conducted by intraflagellar transport (IFT) trains containing the IFT-A and IFT-B complexes that are powered by dynein-2 and kinesin-II motors. We have recently shown that before the assembly of anterograde IFT trains, the IFT-A, IFT-B, and dynein-2 complexes are independently recruited to the mother centriole/basal body. The IFT-B complex, which consists of 16 subunits, can be divided into the IFT-B1 and IFT-B2 subcomplexes, and IFT-B1 can be further divided into the IFT-B1a and IFT-B1b subgroups. Here we investigated how the IFT-B complex is assembled and recruited to the mother centriole for ciliogenesis. Analyses using cells with knockouts of individual IFT-B subunits, and analyses of proteins coimmunoprecipitated with EGFP-fused IFT-B2, IFT-B1b, and IFT-B1a subunits expressed in these knockout cells demonstrated the following: (i) although IFT-B2 is dispensable for the linkage between IFT-B1b and IFT-B1a, it is essential for their localization to the mother centriole; (ii) IFT-B1b is essential both for bridging IFT-B2 and IFT-B1a, and for their localization to the mother centriole; (iii) IFT-B1a is not required for the linkage between IFT-B2 and IFT-B1b nor for their localization to the mother centriole; and (iv) all IFT-B components (IFT-B2, IFT-B1b, and IFT-B1a) are essential for ciliogenesis. Thus, although ciliogenesis is not a prerequisite for the recruitment of the IFT-B complex to the mother centriole, the linkage between IFT-B2 and IFT-B1b is crucial for the mother centriole localization of the IFT-B complex for ciliogenesis.Key words: cilia, ciliogenesis, distal appendages, IFT-B complex, mother centriole.