Gregory J. Hoeprich

Kinesin's Neck-Linker Determines its Ability to Navigate Obstacles on the Microtubule Surface

The neck-linker is a structurally conserved region among most members of the kinesin superfamily of molecular motor proteins that is critical for kinesins processive transport of intracellular cargo along the microtubule surface. Variation in the neck-linker length has been shown to directly modulate processivity in different kinesin families; for example, kinesin-1, with a shorter neck-linker, is more processive than kinesin-2. Although small differences in processivity are likely obscured in vivo by the coupling of most cargo to multiple motors, longer and more flexible neck-linkers may allow different kinesins to navigate more efficiently around the many obstacles, including microtubule-associated proteins (MAPs), that are found on the microtubule surface within cells. We hypothesize that, due to its longer neck-linker, kinesin-2 can more easily navigate obstacles (e.g., MAPs) on the microtubule surface than kinesin-1. We used total internal reflection fluorescence microscopy to observe single-molecule motility from different kinesin-1 and kinesin-2 neck-linker chimeras stepping along microtubules in the absence or presence of two Tau isoforms, 3RS-Tau and 4RL-Tau, both of which are MAPs that are known to differentially affect kinesin-1 motility. Our results demonstrate that unlike kinesin-1, kinesin-2 is insensitive to the presence of either Tau isoform, and appears to have the ability to switch protofilaments while stepping along the microtubule when challenged by an obstacle, such as Tau. Thus, although kinesin-1 may be more processive, the longer neck-linker length of kinesin-2 allows it to be better optimized to navigate the complex microtubule landscape. These results provide new insight, to our knowledge, into how kinesin-1 and kinesin-2 may work together for the efficient delivery of cargo in cells.

The extent of cardiac myosin binding protein-C phosphorylation modulates actomyosin function in a graded manner

Cardiac myosin binding protein-C (cMyBP-C), a sarcomeric protein with 11 domains, C0–C10, binds to the myosin rod via its C-terminus, while its N-terminus binds regions of the myosin head and actin. These N-terminal interactions can be attenuated by phosphorylation of serines in the C1–C2 motif linker. Within the sarcomere, cMyBP-C exists in a range of phosphorylation states, which may affect its ability to regulate actomyosin motion generation. To examine the functional importance of partial phosphorylation, we bacterially expressed N-terminal fragments of cMyBP-C (domains C0–C3) with three of its phosphorylatable serines (S273, S282, and S302) mutated in combinations to either aspartic acids or alanines, mimicking phosphorylation and dephosphorylation respectively. The effect of these C0–C3 constructs on actomyosin motility was characterized in both the unloaded in vitro motility assay and in the load-clamped laser trap assay where force:velocity (F:V) relations were obtained. In the motility assay, phosphomimetic replacement (i.e. aspartic acid) reduced the slowing of actin velocity observed in the presence of C0–C3 in proportion to the total number phosphomimetic replacements. Under load, C0–C3 depressed the F:V relationship without any effect on maximal force. Phosphomimetic replacement reversed the depression of F:V by C0–C3 in a graded manner with respect to the total number of replacements. Interestingly, the effect of C0–C3 on F:V was well fitted by a model that assumed C0–C3 acts as an effective viscous load against which myosin must operate. This study suggests that increasing phosphorylation of cMyBP-C incrementally reduces its modulation of actomyosin motion generation providing a tunable mechanism to regulate cardiac function.

Tau interconverts between diffusive and stable populations on the microtubule surface in an isoform and lattice specific manner

It has been demonstrated that Tau exists on the microtubule lattice in both diffusing and static populations, but how this may relate to Tau function is currently unclear. Tau isoforms are developmentally regulated and have been shown to have disparate effects on microtubule polymerization, the ability to bind microtubules, and the ability to inhibit kinesin. It has also been shown that Tau is sensitive to microtubule stabilizing agents and the ability to affect the persistence length of microtubules and to inhibit kinesin can be altered by stabilizing microtubules with various nucleotide analogs. Given these observations, it is likely the behavior of Tau is dictated by both the isoform of Tau and by structural changes in the microtubule lattice. In the present study, we use single molecule imaging to examine the behavior of the three‐repeat short (3RS) isoform and the four‐repeat long (4RL) isoform on different microtubule tracks stabilized with either paclitaxel or guanylyl‐(α,β)‐methylene‐diphosphate (GMPCPP). On paclitaxel‐stabilized microtubules, we find 3RS‐Tau favors the static conformation and forms complexes consisting of 2–3 molecules, while 4RL‐Tau predominantly exists as a single molecule equally distributed between the static and diffusing populations. However, on GMPCPP‐stabilized microtubules both isoforms favor the diffusing conformation and do not form static complexes composed of more than one Tau molecule. We find both isoforms of Tau interconvert between static and diffusing populations on the microtubule surface, and the equilibrium between these two states depends on both the isoform of Tau and the structure of the underlying microtubule lattice.

Phosphoregulation of Tau modulates inhibition of kinesin-1 motility

Regulation of axonal transport includes control of the microtubule-associated protein Tau. Site-specific pseudophosphorylation of Tau modulates its ability to inhibit kinesin-1 motility by both shifting its static–dynamic state binding equilibrium toward the dynamic state and reducing its affinity for microtubules.

The Axonal Transport Motor Kinesin‐2 Navigates Microtubule Obstacles via Protofilament Switching

Axonal transport involves kinesin motors trafficking cargo along microtubules that are rich in microtubule‐associated proteins (MAPs). Much attention has focused on the behavior of kinesin‐1 in the presence of MAPs, which has overshadowed understanding the contribution of other kinesins such as kinesin‐2 in axonal transport. We have previously shown that, unlike kinesin‐1, kinesin‐2 in vitro motility is insensitive to the neuronal MAP Tau. However, the mechanism by which kinesin‐2 efficiently navigates Tau on the microtubule surface is unknown. We hypothesized that mammalian kinesin‐2 side‐steps to adjacent protofilaments to maneuver around MAPs. To test this, we used single‐molecule imaging to track the characteristic run length and protofilament switching behavior of kinesin‐1 and kinesin‐2 motors in the absence and presence of 2 different microtubule obstacles. Under all conditions tested, kinesin‐2 switched protofilaments more frequently than kinesin‐1. Using computational modeling that recapitulates run length and switching frequencies in the presence of varying roadblock densities, we conclude that kinesin‐2 switches protofilaments to navigate around microtubule obstacles. Elucidating the kinesin‐2 mechanism of navigation on the crowded microtubule surface provides a refined view of its contribution in facilitating axonal transport.