​     Kinesin-5 motors promote spindle dynamics by crosslinking and sliding antiparallel microtubules. The ability of the kinesin-5 proteins to crosslink and slide antiparallel microtubules has been attributed to their unique structure. Several kinesin-5 homologues have been shown to be elongated bipolar homotetramers, with two motor domains positioned at each end of a rod-like structure. Such a bipolar structure can promote the binding of microtubules at both ends of the elongated complex. Since none of the other kinesins function in a homotetrameric complex, the influence of this unique structure on motile properties, ATPase activity, interactions near the catalytic domain, and regulation remains to be explored.

     Domain structure of S. cerevisiae Cin8. (A) The probability of coiled coil formation is plotted against the amino acid number for Cin8 using the COILS program. Other coiled-coil prediction methods such as PAIRCOIL (44) and MULTICOIL gave similar results. (B) Diagram of the domain structure of Cin8. Numbers below the diagram indicate amino acid positions. The highly conserved microtubule binding motor domain is shown in black; the neck is in speckles; four high and moderate probability coiled coils are in light gray; the region that targets Cin8 for APC mediated destruction is in hatched lines; and a region for targeting Cin8 to the nucleus is in black at the C-terminus.
     Co-immunoprecipitation demonstrates that Cin8-Cin8 interactions require coil 1 in the stalk region. Protein extracts were generated from cells co-expressing two forms of Cin8. The first plasmid expresses full length Cin8 with a C-terminal 3HA tag. The second plasmid expresses the indicated truncation or deletion mutant with an N-terminal 6myc tag. The ∆motor-Cin8 mutant has a C-terminal 3HA tag and was co-expressed with full-length 6myc-Cin8. Extracts were treated with 9E10 antibody and protein G beads to precipitate the 6myc-tagged forms of Cin8. The amount of Cin8-3HA that co-immunoprecipitates is shown on the top panel. The bottom panel shows the expression level of 6myc-Cin8 versions prior to immunoprecipitation. Equal quantities of extract were applied to each lane.