Loop L5 determines sensitivity of C. elegans kinesin BMK-1 to the kinesin Eg5-specific inhibitor

The mitotic kinesin Eg5, essential for bipolar spindle formation, is a promising anticancer target. Eg5 features an unusually long loop L5, and specific inhibitors bind to a hydrophobic pocket formed by L5 and the α2/α3 helices, thereby blocking its function. We investigated the nematode kinesin BMK-1, which has a comparably long L5. Caenorhabditis elegans provides an advantageous model for evaluating in vivo effects of kinesin inhibitors. Here, we expressed BMK-1, characterized its biochemical properties, and examined its response to the Eg5-specific inhibitor S-trityl-L-cysteine (STLC). STLC inhibited both ATPase and motility of BMK-1, though less potently than Eg5. An L5-shortened BMK-1 mutant, with loop length reduced to that of conventional kinesins, lost STLC sensitivity while retaining microtubule-stimulated ATPase activity. These findings indicate that BMK-1 and Eg5 share an L5-dependent inhibition mechanism and suggest that Eg5 inhibitors may be applicable to investigating the physiological role of BMK-1 in C. elegans.