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* Cell and Molecular Biology Graduate Group, University of Pennsylvania, Philadelphia, Pennsylvania 19103; CENP-E is a kinesin-like protein that binds
to kinetochores and may provide functions that are
critical for normal chromosome motility during mitosis.
To directly test the in vivo function of CENP-E, we
microinjected affinity-purified antibodies to block the assembly of CENP-E onto kinetochores and then
examined the behavior of these chromosomes. Chromosomes lacking CENP-E at their kinetochores consistently exhibited two types of defects that blocked
their alignment at the spindle equator. Chromosomes
positioned near a pole remained mono-oriented as
they were unable to establish bipolar microtubule
connections with the opposite pole. Chromosomes within the spindle established bipolar connections
that supported oscillations and normal velocities of
kinetochore movement between the poles, but these
bipolar connections were defective because they
failed to align the chromosomes into a metaphase
plate.
Overexpression of a mutant that lacked the amino-terminal 803 amino acids of CENP-E was found to
saturate limiting binding sites on kinetochores and
competitively blocked endogenous CENP-E from assembling onto kinetochores. Chromosomes saturated
with the truncated CENP-E mutant were never found
to be aligned but accumulated at the poles or were
strewn within the spindle as was the case when cells
were microinjected with CENP-E antibodies. As the
motor domain was contained within the portion of
CENP-E that was deleted, the chromosomal defect is
likely attributed to the loss of motor function.
The combined data show that CENP-E provides kinetochore functions that are essential for monopolar
chromosomes to establish bipolar connections and for
chromosomes with connections to both spindle poles to
align at the spindle equator. Both of these events rely
on activities that are provided by CENP-E's motor domain.
Department of
Biology, University of North Carolina, Chapel Hill, North Carolina 27599; and § Fox Chase Cancer Center, Philadelphia,
Pennsylvania 19111
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