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J. Cell Biol.,
Volume 143, Number 4, November 16, 1998 1053-1066
* Department of Cell Biology and We have investigated the mechanism by
which conventional kinesin is prevented from binding
to microtubules (MTs) when not transporting cargo.
Kinesin heavy chain (HC) was expressed in COS cells
either alone or with kinesin light chain (LC). Immunofluorescence microscopy and MT cosedimentation experiments demonstrate that the binding of HC to MTs
is inhibited by coexpression of LC. Association between the chains involves the LC NH2-terminal domain, including the heptad repeats, and requires a region of HC that includes the conserved region of the
stalk domain and the NH2 terminus of the tail domain.
Inhibition of MT binding requires in addition the
COOH-terminal 64 amino acids of HC. Interaction between the tail and the motor domains of HC is supported by sedimentation experiments that indicate that
kinesin is in a folded conformation. A pH shift from 7.2 to 6.8 releases inhibition of kinesin without changing its
sedimentation behavior. Endogenous kinesin in COS cells also shows pH-sensitive inhibition of MT binding.
Taken together, our results provide evidence that a
function of LC is to keep kinesin in an inactive ground
state by inducing an interaction between the tail and
motor domains of HC; activation for cargo transport
may be triggered by a small conformational change that releases the inhibition of the motor domain for MT
binding.
Howard Hughes Medical Institute, Harvard Medical School, Boston, Massachusetts 02115
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