The large loop-8 insert in Saccharomyces cerevisiae kinesin-5 Cin8 is necessary and sufficient to promote noncanonical microtubule interactions

Saccharomyces cerevisiae kinesin-5 Cin8 displays unconventional biochemical behavior including bidirectional motility and ability to bind multiple motor domains per {beta} tubulin dimer in the microtubule lattice. Previous research suggested that a large loop-8 insert near the microtubule binding interface of Cin8 was critical for its noncanonical microtubule binding behavior. Here we utilized mutagenesis, thermodynamic, and kinetic assays to further understand the mechanism for how this loop-8 insert promotes super-stoichiometric microtubule binding in Cin8. This loop-8 insert that interrupts the conserved {beta}5a/b hairpin was swapped between Cin8, Eg5 (KIF11, a human kinesin-5) and Kip1 (another S. cerevisiae kinesin-5). Cin8 with the loop-8 insert from Eg5 (Cin8-EL8) binds one motor per tubulin dimer, whereas Eg5 with the loop-8 insert from Cin8 (Eg5-CL8) binds approximately 2-4 motors per tubulin dimer. Eg5-CL8 bound the canonical and noncanonical sites on the microtubule lattice with weakened oligomerization between motors, while Cin8-EL8 showed only canonical site binding. These results demonstrate that the large loop-8 insert in Cin8 is necessary and sufficient to promote noncanonical microtubule binding behavior.