|Dynein Motor Domain Shows Ring-Shaped Motor, Buttress|
Movement is fundamental to life. It takes place even at the cellular level where cargo is continually being transported by motor proteins. These tiny machines convert the energy gained from hydrolysing ATP into a series of small conformational changes that allow them to literally “walk” along microscopic tracks. Motor proteins (in the kinesin and myosin families) have been extensively studied by x-ray crystallography, but until recently there was little molecular structural information for dyneins, another type of motor protein. A group from the University of California, San Francisco, working at ALS Beamline 8.3.1 has reported the 6-Å-resolution structure of the motor domain of dynein in yeast. It reveals details of the ring-shaped motor as well as a new, unanticipated feature called the buttress that may play an important role in dynein’s mechanical cycle.
Like other motor proteins, dynein produces movement by coupling ATP binding and hydrolysis with changes in shape. ATP binds to a dynein’s motor domain, causing it to release from the microtubule track to which it’s bound and resetting a mobile part of the structure called the linker domain into a different position. Then dynein rebinds the microtubule, triggering the hydrolysis of ATP and a conformational change in the position of the linker domain, generating force. Finally ADP is released from the motor domain, allowing another ATP molecule to bind.
Dynein differs from other motor proteins in two ways. Instead of having a single ATP-binding domain, dynein comprises a ring of six AAA+ domains (ATPases Associated with diverse cellular Activities). At least two of these domains are sites of hydrolysis, which raises the question of how many ATP molecules dynein actually uses. Second, the microtubule-track binding site is in a separate domain from the ATP binding site, found at the end of a long alpha helical projection called the stalk.
The structure of dynein’s microtubule-binding domain was previously determined at ALS Beamline 8.3.1 (see ALS Science Highlight How Dynein Binds to Microtubules). Now, the x-ray crystal structure for its motor domain has been solved at the same beamline using phases determined from a polytungstate cluster (W12) heavy-atom derivative. Because of the conserved structure of AAA domains and the high alpha helix content of the dynein, researchers were able to build a model and assign a position to all the secondary structure elements. This model shows the mobile linker domain arching over the ring of AAA domains. It also shows that the linker is on the same face of the ring as a set of highly conserved inserts in the AAA domains, suggesting that as the linker moves it may contact different AAA domains. The ring itself was remarkably asymmetric, with some AAA domains packed close together and others gaping open. Notably, dynein’s main ATP binding site, between AAA1 and AAA2, was in an open conformation, consistent with the fact that the structure was solved in the absence of ATP and ADP.
An intriguing part of the newly modeled structure is the buttress: a coiled-coil hairpin that extends out of AAA5 to contact the microtubule-binding stalk. The presence of the buttress was unanticipated even though, in hindsight, both electron microscopy and coiled-coil prediction software previously hinted at its existence. It is perfectly placed to link ATP-driven rearrangements of the AAA ring to conformational changes that propagate along the stalk to change the affinity of the microtubule-binding domain for its track. The buttress therefore seems to be a key element in the coupling of ATP hydrolysis to dynein’s movement along microtubules.
Research conducted by A.P. Carter (UC San Francisco and Medical Research Council Library of Molecular Biology) and C. Cho, L. Jin, and R.D. Vale (UC San Francisco).
Research funding: U.S. Department of Energy (DOE), Office of Basic Energy Sciences (BES). Operation of the ALS is supported by DOE BES.
Publication about this research: A.P. Carter, C. Cho, L. Jin, and R.D. Vale, "Crystal structure of the dynein motor domain," Science 331, 6021 (2011).
ALS Science Highlight #238