Harnessing the power of cryo electron microscopy to visualise a full-length kinesin KIFC1 (HSET)

Abstract

Cancer cells are known to exhibit a phenomenon typically known as centrosome amplification (CA), which is responsible for multipolar spindle formation during mitosis. This severely impairs cytokinesis and could hence prove fatal to the cells. However, in these cancer cells, a specific microtubule-associated motor protein KIFC1 (also known as HSET), which has the ability to cluster centrosomes, is overexpressed, thereby resolving the spindle multipolarity and facilitating bipolar mitosis. Recent studies have revealed that HSET knock-down renders the CA-positive cancer cells non-viable, while having bo effect on the viability of non-cancerous cells. This specificity makes HSET an attractive target for anti-cancer drug discovery. HSET is a minus-end directed motor and a member of the kinesin-14A family. It has an N-terminal cargo-binding domain followed by a long coiled coil stalk connected to a C-terminal catalytic motor domain. While a crystal structure of the motor domain of HSET bound to ADP has been reported, the inherent flexibility of the coiled coil and cargo-binding domains has rendered crystallization attempts of the full-length protein unsuccessful. Thus, little is known about the global structural changes that result from the communication between the three domains of HSET. My aim is to determine a full-length structure of HSET in its Apo and ADP-bound forms using cryo electron microscopy combined with complementary biophysical characterization. Here I report 3D reconstructions of the Apo- and ADP-bound full-length HSET at a resolution of 20 A using negative stain EM. An ADP-HSET structure was determined at a resolution of 8.5 A by cryo EM suing a Volta Phase Plate. The maps provide the first structural insight into the HSET homodimer and show the motor domains in an asymmetric arrangement with respect to the coiled coil stalk. The maps highlight two unique conformations of Apo-HSET and a single conformer of HSET in its ADP-bound form.