Distinct roles of 1α and 1β heavy chains of the inner arm dynein I1 of Chlamydomonas flagella

The Chlamydomonas I1 dynein is a two-headed inner dynein arm important for the regulation of flagellar bending. Here we took advantage of mutant strains lacking either the 1 alpha or 1 beta motor domain to distinguish the functional role of each motor domain. Single-particle electronic microscopic analysis confirmed that both the I1 alpha and I1 beta complexes are single headed with similar ringlike, motor domain structures. Despite similarity in structure, however, the I1 beta complex has severalfold higher ATPase activity and microtubule gliding motility compared to the I1a complex. Moreover, in vivo measurement of microtubule sliding in axonemes revealed that the loss of the 1 beta motor results in a more severe impairment in motility and failure in regulation of microtubule sliding by the I1 dynein phosphoregulatory mechanism. The data indicate that each I1 motor domain is distinct in function: The I1 beta motor domain is an effective motor required for wild-type microtubule sliding, whereas the I1 alpha motor domain may be responsible for local restraint of microtubule sliding.