Discrete PIH proteins function in the cytoplasmic preassembly of different subsets of axonemal dyneins.

Axonemal dyneins are preassembled in the cytoplasm before being transported into cilia and flagella. Recently, PF13/KTU, a conserved protein containing a PIH (protein interacting with HSP90) domain, was identified as a protein responsible for dynein preassembly in humans and Chlamydomonas reinhardtii. This protein is involved in the preassembly of outer arm dynein and some inner arm dyneins, possibly as a cofactor of molecular chaperones. However, it is not known which factors function in the preassembly of other inner arm dyneins. Here, we analyzed a novel C. reinhardtii mutant, ida10, and found that another conserved PIH family protein, MOT48, is responsible for the formation of another subset of inner arm dyneins. A variety of organisms with motile cilia and flagella typically have three to four PIH proteins, including potential homologues of MOT48 and PF13/KTU, whereas organisms without them have no, or only one, such protein. These findings raise the possibility that multiple PIH proteins are commonly involved in the preassembly of different subsets of axonemal dyneins.