Modulation of inner junction proteins contributes to axoneme differentiation.

Cilia build distinct subdomains with variable axonemal structures to perform diverse functions in cell motility and signaling. In sensory cilia across species, an axoneme differentiates longitudinally into a middle segment with nine microtubule (MT) doublets and a distal segment with nine MT singlets that extends from the A tubules of the doublets. Here, we study axoneme differentiation in Caenorhabditis elegans by analyzing the flagellar inner junction protein FAP20 and PCRG1 that connect A and B tubules in Chlamydomonas . The nematode CFAP-20 is restricted to the middle segment with doublets, and its loss disconnects A and B tubules. However, PCRG-1 is absent from most sensory cilia, and its deletion does not disrupt cilia. Ectopic introduction of PCRG-1 into cilia generated abnormal MT doublets in the distal segment and reduced intraflagellar transport and animal sensation. Thus, the absence of an inner junction protein prevents B-tubule extension, which contributes to axoneme differentiation and ciliary function.