The conversion of chemical energy into mechanical force by AAA+ (ATPases associated with diverse cellular activities) ATPases is integral to cellular processes, including DNA replication, protein unfolding, cargo transport, and membrane fusion1. The AAA+ ATPase motor cytoplasmic dynein regulates ciliary trafficking2, mitotic spindle formation3, and organelle transport4, and dissecting its precise functions has been challenging due to its rapid timescale of action and the lack of cell-permeable, chemical modulators. Here we describe the discovery of ciliobrevins, the first specific small-molecule antagonists of cytoplasmic dynein. Ciliobrevins perturb protein trafficking within the primary cilium, leading to their malformation and Hedgehog signaling blockade. Ciliobrevins also prevent spindle pole focusing, kinetochore-microtubule attachment, melanosome aggregation, and peroxisome motility in cultured cells. We further demonstrate the ability of ciliobrevins to block dynein-dependent microtubule gliding and ATPase activity in vitro. Ciliobrevins therefore will be useful reagents for studying cellular processes that require this microtubule motor and may guide the development of additional AAA+ ATPase superfamily inhibitors.

The ribosome accelerates the rate of peptide bond formation by at least 10(7)-fold, but the catalytic mechanism remains controversial. Here we report evidence that a functional group on one of the tRNA substrates plays an essential catalytic role in the reaction. Substitution of the P-site tRNA A76 2' OH with 2' H or 2' F results in at least a 10(6)-fold reduction in the rate of peptide bond formation, but does not affect binding of the modified substrates. Such substrate-assisted catalysis is relatively uncommon among modern protein enzymes, but it is a property predicted to be essential for the evolution of enzymatic function. These results suggest that substrate assistance has been retained as a catalytic strategy during the evolution of the prebiotic peptidyl transferase center into the modern ribosome.

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Although assembly of the mitotic spindle is known to be a precisely controlled process, regulation of the key motor proteins involved remains poorly understood. In eukaryotes, homotetrameric kinesin-5 motors are required for bipolar spindle formation. Eg5, the vertebrate kinesin-5, has two modes of motion: an adenosine triphosphate (ATP)–dependent directional mode and a diffusive mode that does not require ATP hydrolysis. We use single-molecule experiments to examine how the switching between these modes is controlled. We find that Eg5 diffuses along individual microtubules without detectable directional bias at close to physiological ionic strength. Eg5's motility becomes directional when bound between two microtubules. Such activation through binding cargo, which, for Eg5, is a second microtubule, is analogous to known mechanisms for other kinesins. In the spindle, this might allow Eg5 to diffuse on single microtubules without hydrolyzing ATP until the motor is activated by binding to another microtubule. This mechanism would increase energy and filament cross-linking efficiency.