Heterodimeric Ciliary Dynein f/I1 Adopts a Distinctive Structure, Providing Insight Into the Autoinhibitory Mechanism Common to the Dynein Family.

Dyneins are huge motor protein complexes that are essential for cell motility, cell division, and intracellular transport. Dyneins are classified into three major subfamilies, namely cytoplasmic, intraflagellar-transport (IFT), and ciliary dyneins, based on their intracellular localization and functions. Recently, several near-atomic resolution structures have been reported for cytoplasmic/IFT dyneins. In contrast, the structures of ciliary dyneins, as well as their regulatory mechanisms, have yet to be fully elucidated. Here, we isolated a heterodimeric ciliary dynein (IDA-f/I1) from Chlamydomonas reinhardtii, a ciliated green alga, and studied its structure in the presence or absence of ATP by negative-stain electron microscopy and single-particle analysis. Surprisingly, a population of IDA-f adopted a distinctive compact structure, which has been scarcely reported for ciliary dyneins but is very similar to the "phi-particle" structure widely recognized as the autoinhibited/inactivated conformation for cytoplasmic/IFT dyneins. Our results suggest that the inactivation mechanism of dimeric dyneins is conserved in all three dynein subfamilies, regardless of their cellular functions, highlighting the intriguing intrinsic regulatory mechanism that may have been acquired at an early stage in the evolution of dynein motors.