Loss of Kif18A Results in Spindle Assembly Checkpoint Activation at Microtubule-Attached Kinetochores

The spindle assembly checkpoint (SAC) halts anaphase progression until all kinetochores have obtained bipolar, stable attachments to the mitotic spindle. Upon initial attachment, chromosomes undergo oscillatory movements to reach metaphase. Once a chromosome is correctly attached and positioned, these oscillatory movements are reduced by the motor protein Kifl 8A, and loss of Kifl 8A results in chromosome hyper-oscillations. By using a haploid genetic approach, we found that loss of Kifl 8A is lethal in wild-type human HAP1 cells, but not in SAC-deficient HAP1 cells. Unexpectedly, we found that the hyper-oscillations after Kifl 8A loss are not associated with chromosome missegregations. Rather, we found that loss of KiflE3A results in a loss of tension across a subset of kinetochores accompanying SAC activation. Strikingly, the SAC active kinetochores appear to have established fully functional kinetochore-microtubule (k-Mt) attachments, allowing proper chromosome segregation. These findings shed new light on the role of Kifl 8A in chromosome segregation and demonstrate that the SAC can be activated at kinetochores that are occupied by fully functional k-Mts that lack tension.