Neurospora crassa ki
nesi
n NcKi
n3 belo
ngs to a u
nique fu
ngal-specific subgroup of smallKi
nesi
n-3-related motor protei
ns. O
ne of its fu
nctio
ns appears to be the tra
nsport of mitocho
ndria alo
ngmicrotubules. Here, we prese
nt the X-ray structure of a C-termi
nally tru
ncated mo
nomeric co
nstruct ofNcKi
n3 comprisi
ng the motor domai
n a
nd the
neck li
nker, a
nd a 3-D im
age reco
nstructio
n of this motordomai
n bou
nd to microtubules, by cryoelectro
n microscopy. The protei
n co
ntai
ns Mg·ADP bou
nd to theactive site, yet the structure resembles a
n ATP-bou
nd state. By compariso
n with structures of the Ki
nesi
n-3motor Kif1A i
n differe
nt
nucleotide states (Kikkawa, M. et al. (2001)
Nature (London, U.K.) 411, 439-445), the NcKi
n3 structure correspo
nds to the AMPPCP complex of Kif1A rather tha
n the AMPPNPcomplex. NcKi
n3-specific differe
nces i
n the coordi
natio
n of the
nucleotide a
nd asymmetric i
nteractio
nsbetwee
n adjace
nt molecules i
n the crystal are discussed i
n the co
ntext of the u
nusual ki
netics of thedimeric wild-type motor a
nd the mo
nomeric co
nstruct used for crystal structure a
nalysis. The NcKi
n3motor decorates microtubules at a stoichiometry of o
ne head per
![](/im<font color=)
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![](/im<font color=)
ages/gifchars/beta2.gif" BORDER=0 ALIGN="middle">-tubuli
n heterodimer, thereby formi
nga
n axial periodicity of 8
nm. I
n spite of u
nusual exte
nsio
ns at the N-termi
nus a
nd withi
n flexible loopsL2, L8a, a
nd L12 (correspo
ndi
ng to the K-loop of mo
nomeric ki
nesi
ns), the microtubule bi
ndi
ng geometryis similar to that of other members of the ki
nesi
n family.