178 Y.-R.J.Lee·B.Liu
Fig. 4Organization of spindle midzone microtubules.AAn anti-tubulin immunofluores-
cent image showing microtubule organization in an anaphase onion root cell. Midzone
microtubules are indicated byarrowheads, and remnants of shortening kinetochore mi-
crotubules are indicated byarrows.BKinesin-5 members are proposed to play a role in
sliding anti-parallel microtubules apart. Dumbbell-shaped homotetramers of Kinesin-5
are plus end directed motors, and its directionality is indicated byarrows
Members of the BIMC/Kinesin-5 subfamily are the key players for micro-
tubule sliding in the spindle (Sharp et al. 2000c). BIMC was first isolated from
the filamentous fungusAspergillus nidulansas a motor for spindle pole body
separation (Enos and Morris 1990). It contains an N-terminal motor domain
followed by long coiled-coils. Towards the C-terminus of Kinesin-5, there is
a signature domain known as the BIMC box with a CDK phosphorylation site,
indicating its activity is regulated in a cell cycle-dependent fashion (Blangy et al.
1995). Biochemical analysis of its Drosophila homolog indicates that the native
form of such kinesins is a homotetramer with two motor domains at each dis-
tal end in a dumbbell-like shape so that it is often referred to as bipolar kinesin
(Kashina et al. 1996). Kinesin-5/BIMC is essential for maintaining the bipolar
structure of the spindle as its inactivation leads to collapse of the bipolar spindle
into a monopolar one (Blangy et al. 1995; Enos and Morris 1990).