Proteins involved in mitosis are attractive potential targets for cancer therapy, as their inhibition may allow the specific targetting of proliferating cells (Bergnes et al.
; Harrison et al.
). Indeed, there are a number of antimitotic drugs in clinical use, all of which target microtubule (MT) dynamics. Unfortunately, however, these drugs show significant side effects, as MTs are involved in a wide variety of cellular processes aside from mitosis.
Kinesin motor proteins move along MTs in an ATP-dependent manner. While they are conventionally thought to function in cellular cargo transport, a number of kinesins have been found to act during mitosis (Wordeman, 2010
), making them potential targets for antimitotic drugs (Good et al.
). In fact, some of the mitotic kinesins appear to function exclusively during mitosis and as such they may deliver on the promise of an improved side-effect profile in anticancer therapy through inhibition of mitotic proteins.
Human Eg5 (KSP, kinesin spindle protein, KIF11), a member of the kinesin-5 family (Miki et al.
), is a well characterized mitotic kinesin that is required to establish a bipolar mitotic spindle. Eg5 forms homotetramers that can attach to neighbouring antiparallel spindle MTs and slide them against each other, thus separating the duplicated centrosomes (Kapitein et al.
Loss of Eg5 function owing to RNA interference or small-molecule inhibitors results in the formation of monoastral spindles, cell-cycle arrest and apoptosis (Blangy et al.
; Weil et al.
; Mayer et al.
). A number of small-molecule inhibitors of Eg5 have been identified, including MK-0731, pyrrolotriazin-4-one-based inhibitors and the quinazolin-4-one-based ispinesib (Cox & Garbaccio, 2010
; Lad et al.
; Kim et al.
), all of which are allosteric inhibitors that bind to the unique L5 loop region of the catalytic domain. Ispinesib (also named SB-715992 or CK0238273) is a potent and selective inhibitor of Eg5 that is currently in multiple phase II clinical trials (Burris et al.
; Souid et al.
) and is one of the most advanced drug candidates. The importance of the quinazolin-4-one scaffold targeting Eg5 is further underlined by the fact that three structurally related compounds are in various stages of clinical development: SB-743921, a second-generation ispinesib analogue (Holen et al.
), AZD4877 (Esaki et al.
) and Arq621 (Chen et al.
). A greater understanding of the molecular details of the protein–inhibitor interactions of this class of compounds is therefore crucial.
Although an Eg5–ispinesib complex has been reported previously (Zhang et al.
), no coordinates or experimental data were made available, which hampers detailed analysis of this important enzyme–inhibitor interaction and the use of the complex for further structure-guided design. Here, we report the 2.6 Å resolution structure of the ternary complex of the Eg5 motor domain in complex with Mg2+
ADP and ispinesib. The structure provides a detailed overview of the interaction between ispinesib and the Eg5 motor domain and a rationale for further drug development.