Microtubule-regulating kinesins

Kinesins can regulate microtubule dynamics? The conventional function of kinesins, much like a molecular freight train, is to transport cargo by motoring along a microtubule (MT) track. But there exists another class of kinesins whose job relates more to track maintenance than transportation. These ‘regulatory kinesins’ modify the track on which they walk in order to shape the MT cytoskeleton. Regulatory kinesins control MT assembly and/or disassembly in order to influence the organization and dynamics of MT-based cellular machines. In other words, regulatory kinesins reconfigure the layout of the ‘rail map’.How many kinesins do this? Of the ∼45 kinesins encoded by the human genome, 9 are known to regulate microtubule dynamics. Microtubule-regulating kinesins stratify into three basic classes: elongases, pause factors, and depolymerases (Figure 1Figure 1). The kinesin-7 CENP-E has been shown to promote microtubule elongation, suggesting that it may function as an elongase. kinesin-4s and -8s function as pause, or assembly-attenuating, factors and this class includes the mammalian motors Kif4/Xklp1 (a kinesin-4) and Kif18A (a kinesin-8). The largest and most studied class of regulatory kinesins, the depolymerases, is made up of members of the kinesin-8, -13, and -14 families. Specific examples include MCAK/Kif2C (a kinesin-13), yeast Kip3 (a kinesin-8) and Kar3 (a kinesin-14).Figure 1Microtubule-regulating kinesins.Regulatory kinesins partition into three classes. Depolymerases (red) promote microtubule disassembly. Elongases (green) promote microtubule assembly. Pause factors (blue) suppress the inherent dynamicity of microtubules.View Large Image | View Hi-Res Image | Download PowerPoint SlideIt is worth noting that some kinesins indirectly impact MT dynamics (e.g., kinesin-1s promote MT elongation by activating JNK and delivering MT assembly factors to plus-ends). However, this Quick Guide focuses on kinesins whose motor activity directly alters MT dynamics.When did they start doing that? Evolutionarily, regulatory kinesins are conserved throughout the eukaryotic kingdom. Two out of six kinesins (Kar3 and Kip3) in budding yeast directly regulate MT dynamics. Interestingly, one eukaryote, Theileria annulata, encodes just two kinesins — a kinesin-8 and a kinesin-13 — suggesting that microtubule regulation is an ancient and critical function of kinesin-like motors (Claire Walczak, personal communication). Historically, kinesins were first discovered to have MT-regulating capabilities in the mid 1990s. It is reasonable to assume that the list of regulatory kinesins is incomplete, as regulatory functions are still being uncovered for specific kinesins. For example, the ability of Kif19 to depolymerize MTs was discovered last year.What cellular processes do these motors control? As evidenced by their conservation from yeast to human, regulatory kinesins contribute to essential cellular processes. You can bet that regulatory kinesins are involved any time a cellular activity capitalizes on MT dynamics. One job of regulatory kinesins is to build MT-based cellular machines, such as the mitotic spindle. Compared to the interphase array, the spindle is composed of a copious amount of short dynamic MTs. Upon mitotic entry, depolymerases like Kif2A and MCAK must ramp up the MT catastrophe frequency in order to shift the distribution of MT number and length. In this manner, regulatory kinesins contribute to the massive rearrangement of the MT cytoskeleton necessary for spindle assembly.In addition to assembling MT-based structures, regulatory kinesins fine-tune the dynamics of these cellular machines for optimized performance. In the mitotic spindle, for example, Kif18A dampens kinetochore-MT dynamics to prevent excessive chromosome movement and MCAK dismantles flawed kinetochore-MT attachments. In mouse epithelial cells, the kinesin-8 Kif19 prevents excessive elongation of motile cilia. And kinesin-13s in Giardia and Leishmania coordinate with intraflagellar transport machinery to control cilia length.Each of these examples demonstrate how cells utilize regulatory kinesins to modulate the dynamic instability inherent to MTs in order to fine-tune cellular processes involving the cytoskeleton.Why use motors when the cell contains so many other regulators of microtubule dynamics? Actually, some scientists speculate that kinesins were originally selected for their ability to regulate MT dynamics, and that motility evolved later. This is because dynamic polymers preceded motor proteins evolutionarily, so proteins that bound these polymers might have coupled to polymer dynamics initially. Nonetheless, there are advantages to having a motor domain on a regulatory factor. For example, the processivity of Kif18A is exquisitely tuned to get the motor to the plus-ends of kinetochore fibers, which are built from spindle microtubules that attach to kinetochores. This enables Kif18A to ‘measure’ MT length, so that long MTs are affected more than short MTs.How do kinesins regulate microtubule dynamics? In general, depolymerases can shrink MTs by removing tubulin from MT ends or, for dynamic MTs, by suppressing subunit addition. The latter mechanism probably works because it promotes loss of the GTP-tubulin cap. Similarly, elongases can elongate MTs by adding tubulin to MT ends or by preventing catastrophes. The mechanistic details of these activities are poorly understood, but it is appreciated that regulatory kinesins can utilize their motor domains to alter the structure of MT protofilaments. MCAK, the best-described regulatory kinesin, stabilizes the bent conformation of protofilaments that predisposes MTs to depolymerization. In contrast, CENP-E has been proposed to stabilize the straight conformation of protofilaments that predisposes MTs to polymerization.But how does one kinesin use its motor domain to move while another uses it to manipulate MT dynamics? While all kinesins share a similar motor domain, structural differences can lead to variations of the ATPase cycle and create additional tubulin contact points. For example, the mechanochemical cycle of MCAK is limited by ATP hydrolysis rather than ADP release. This results in MCAK diffusing along the MT lattice instead of walking. The MT end stimulates MCAK to exchange nucleotides, transitioning the motor to a tightly bound state. Taken together, these unique features of the MCAK motor domain tune it to identify and stabilize curved protofilaments at MT ends. Notably, our current knowledge on how kinesins alter MT dynamics is heavily influenced by work on kinesin-13s, largely because mechanistic details of how other kinesins work do not exist. We await further studies to see if the kinesin-13 paradigm is universal, or if other kinesins use unique biochemistries to shape the MT cytoskeleton.