Vesicular transport is a dynamic process that requires coordinated interactions between membrane and cytoskeleton. The mechanisms and molecules integrating these interactions are unclear. A Rho protein, RhoD, might provide a molecular link between membrane traffic and the cytoskeleton. Activated RhoD causes rearrangements of the actin cytoskeleton and cell surface, and governs early endosome motility and distribution.

Conventional kinesin is a molecular motor consisting of an N-terminal catalytic motor domain, an extended stalk and a small globular C-terminus. Whereas the structure and function of the catalytic motor domain has been investigated, little is known about the function of domains outside the globular head. A short coiledcoil region adjacent to the motor domain, termed the neck, is known to be important for dimerization and may be required for kinesin processivity. We now provide evidence that a helix-disrupting hinge region (hinge 1) that separates the neck from the first extended coiled-coil of the stalk plays an essential role in basic motor activity. A fast fungal kinesin from Syncephalastrum racemosum was used for these studies. Deletion, substitution by a coiled-coil and truncation of the hinge 1 region all reduce motor speed and uncouple ATP turnover from gliding velocity. Insertion of hinge 1 regions from two conventional kinesins, Nkin and DmKHC, fully restores motor activity, whereas insertion of putative flexible linkers of other proteins does not, suggesting that hinge 1 regions of conventional kinesins can functionally replace each other. We suggest that this region is essential for kinesin movement in its promotion of chemo-mechanical coupling of the two heads and therefore the functional motor domain should be redefined to include not only the catalytic head but also the adjacent neck and hinge 1 domains.

Conventional kinesins are molecular motors that move towards the plus end of microtubules. In animal species, they have been shown to be remarkably conserved in terms of both their primary sequence and several physiological properties, including their velocity of movement. Here we report the cloning of Synkin, a homologue of conventional kinesin from the zygomycete fungus Syncephalastrum racemosum [Steinberg, Eur. J. Cell Biol. 73 (1997) 124^131] that is 4^5 times faster than its animal counterparts. Expression in bacteria yields a fully functional motor that moves at the same speed as the native motor isolated from fungal hyphae and has similar hydrodynamic properties. Its sequence is most closely related to that of two other fungal kinesins from Neurospora and Ustilago, and shares several biochemical properties with the Neurospora motor. Fungal kinesins therefore seem to form a conserved subfamily of conventional kinesins distantly related to animal kinesins. They may help to identify sequence features important for determining motor velocity.z 1998 Federation of European Biochemical Societies.