Control of axonemal inner dynein arms

In this chapter, we focus on the assembly, organization, and regulation of the axonemal inner dynein arms and their role in ciliary/flagellar motility. In contrast to the outer dynein arms, which are homogeneous, there are seven major and four minor species of inner dynein arms. Each inner dynein arm is distinct in composition and targeted to a precise and unique location within the 96-nm axonemal repeat. These conclusions are based on biochemical and ultrastructural analysis of Chlamydomonas mutants lacking subsets of dyneins. The motility phenotypes of these Chlamydomonas mutants also revealed the importance of the inner dynein arms for control of the size and shape of the axonemal bend, features of motility referred to as waveform. We review data revealing that second messengers, including calcium and cyclic nucleotides, can control ciliary motility by modulation of dynein activity. In particular, we focus on I1/f dynein and its regulation by a conserved phosphorylation pathway that includes signals from the central pair, radial spoke, and a network of axonemal kinases and phosphatases that are physically located in the axoneme.