Neurospora crassa kinesin NcKin3 belongs to a unique fungal-specific subgroup of smallKinesin-3-related motor proteins. One of its functions appears to be the transport of mitochondria alongmicrotubules. Here, we present the X-ray structure of a C-terminally truncated monomeric construct ofNcKin3 comprising the motor domain and the nec
k lin
ker, and a 3-D image reconstruction of this motordomain bound to microtubules, by cryoelectron microscopy. The protein contains Mg·ADP bound to theactive site, yet the structure resembles an ATP-bound state. By comparison with structures of the Kinesin-3motor Kif1A in different nucleotide states (Ki
kkawa, M. et al. (2001)
Nature (London, U.K.) 411, 439-445), the NcKin3 structure corresponds to the AMPPCP complex of Kif1A rather than the AMPPNPcomplex. NcKin3-specific differences in the coordination of the nucleotide and asymmetric interactionsbetween adjacent molecules in the crystal are discussed in the context of the unusual
kinetics of thedimeric wild-type motor and the monomeric construct used for crystal structure analysis. The NcKin3motor decorates microtubules at a stoichiometry of one head per
-tubulin heterodimer, thereby formingan axial periodicity of 8 nm. In spite of unusual extensions at the N-terminus and within flexible loopsL2, L8a, and L12 (corresponding to the K-loop of monomeric
kinesins), the microtubule binding geometryis similar to that of other members of the
kinesin family.