Electron Microscopic Imaging and Analysis of Isolated Dynein Particles

The axoneme, a biological template to design a swell energy recovery system

The axonemal micro-machinery, the axial skeleton and actuator of cilia and flagella of eukaryotic cells, is able to bend and twist and generates wave trains. We already demonstrated that it can be the template to construct an active trunk robot (Cibert 2013 Bioinspir. Biomim. 8 026006). The work presented in this paper describes how the axonemal model can also be useful to design worm shaped devices to convert swell energy into usable energy such as electricity. Using dynamic simulation we have designed three models either very close to the biological machinery or being a reasonable interpretation of its functioning principles. Their main advantage as compared with another existing device, Pelamis, is the additional twist degree-of-freedom that gives interesting properties to the system.

Lis1 restricts the conformational changes in cytoplasmic dynein on microtubules

Cytoplasmic dynein is a microtubule-based motor protein that transports intracellular cargo and performs various functions during cell division. We previously reported that Lis1 suppressed dynein motility on microtubules in an idling state. Recently, a model showed that Lis1 prevents the ATPase domain of dynein from transmitting a detachment signal to its microtubule-binding domain. However, conformational information on dynein is limited. We used electron microscopy to investigate the conformation of dynein and nucleotide-induced conformational changes on microtubules. The conformation of dynein differed depending on the presence or absence of a nucleotide. In the presence of the nucleotide ADP-vanadate, dynein displayed an extended form on microtubules (extended form), whereas in the absence of a nucleotide, dynein lay along microtubules (compact form). This conformational change reflects chemomechanical coupling in dynein walking on microtubules. We also found that Lis1 fixed the conformation of dynein in the compact form regardless of the nucleotide condition. Removal of the Lis1 dimerization motif abolished Lis1-dependent fixation of dynein in the compact form. This suggests that the idling state of dynein on microtubules induced by Lis1 occurs through the Lis1-dependent arrest of dynein chemomechanical coupling.

Slow axonemal dynein e facilitates the motility of faster dynein c

We highly purified the Chlamydomonas inner-arm dyneins e and c, considered to be single-headed subspecies. These two dyneins reside side-by-side along the peripheral doublet microtubules of the flagellum. Electron microscopic observations and single particle analysis showed that the head domains of these two dyneins were similar, whereas the tail domain of dynein e was short and bent in contrast to the straight tail of dynein c. The ATPase activities, both basal and microtubule-stimulated, of dynein e (kcat = 0.27 s(-1) and kcat,MT = 1.09 s(-1), respectively) were lower than those of dynein c (kcat = 1.75 s(-1) and kcat,MT = 2.03 s(-1), respectively). From in vitro motility assays, the apparent velocity of microtubule translocation by dynein e was found to be slow (Vap = 1.2 ± 0.1 μm/s) and appeared independent of the surface density of the motors, whereas dynein c was very fast (Vmax = 15.8 ± 1.5 μm/s) and highly sensitive to decreases in the surface density (Vmin = 2.2 ± 0.7 μm/s). Dynein e was expected to be a processive motor, since the relationship between the microtubule landing rate and the surface density of dynein e fitted well with first-power dependence. To obtain insight into the in vivo roles of dynein e, we measured the sliding velocity of microtubules driven by a mixture of dynein e and c at various ratios. The microtubule translocation by the fast dynein c became even faster in the presence of the slow dynein e, which could be explained by assuming that dynein e does not retard motility of faster dyneins. In flagella, dynein e likely acts as a facilitator by holding adjacent microtubules to aid dynein c's power stroke.

Distinct roles of 1alpha and 1beta heavy chains of the inner arm dynein I1 of Chlamydomonas flagella

We took advantage of Chlmaydomonas flagellar mutant strains lacking either the 1α or 1β motor domain in I1 dynein to distinguish the functional role of each. The 1β motor domain is an effective motor required for control of microtubule sliding, whereas the 1α motor domain may restrain microtubule sliding driven by other dyneins.

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 1α or 1β motor domain to distinguish the functional role of each motor domain. Single- particle electronic microscopic analysis confirmed that both the I1α and I1β complexes are single headed with similar ringlike, motor domain structures. Despite similarity in structure, however, the I1β complex has severalfold higher ATPase activity and microtubule gliding motility compared to the I1α complex. Moreover, in vivo measurement of microtubule sliding in axonemes revealed that the loss of the 1β 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 I1β motor domain is an effective motor required for wild-type microtubule sliding, whereas the I1α motor domain may be responsible for local restraint of microtubule sliding.