We previously demonstrated that PACRG plays a role in regulating dynein-driven microtubule sliding in motile cilia. To expand our understanding of the role of PACRG in ciliary assembly and motility, we used a combination of functional and structural studies, including newly identified Chlamydomonas pacrg mutants. Using cryo-electron tomography we show that PACRG and FAP20 form the inner junction between the A- and B-tubule along the length of all nine ciliary doublet microtubules. The lack of PACRG and FAP20 also results in reduced assembly of inner-arm dynein IDA b and the beak-MIP structures. In addition, our functional studies reveal that loss of PACRG and/or FAP20 causes severe cell motility defects and reduced in vitro microtubule sliding velocities. Interestingly, the addition of exogenous PACRG and/or FAP20 protein to isolated mutant axonemes restores microtubule sliding velocities, but not ciliary beating. Taken together, these studies show that PACRG and FAP20 comprise the inner junction bridge that serves as a hub for both directly modulating dyneindriven microtubule sliding, as well as for the assembly of additional ciliary components that play essential roles in generating coordinated ciliary beating.
Bibliographical noteFunding Information:
We thank Douglas Tritscher (University of Minnesota) for his contribution to the sequence analysis of the pf12 strains. We thank Daniel Stoddard (UT Southwestern Medical Center) for management of the electron microscope facilities and training. The UT Southwestern Cryo-Electron Microscopy Facility is supported in part by the CPRIT Core Facility Support Award RP170644. The three-dimensional averaged structures in this study have been deposited in a public database, namely EMDataResource, with the accession codes EMD-0628, EMD-0629 and EMD-0630. This study was funded by the following grants: National Institutes of Health R01GM112050 to E.F.S., R01GM083122 to D.N., and R01GM055667 to M.P.