Architecture of the dynein adaptor complex Fts-Hook3-FHIP1B.

Abstract

Organisation of the cell's diverse cargo is dependent on the opposing microtubule-based motors kinesin and dynein. Hook3 is a highly conserved cargo adaptor that can simultaneously scaffold kinesin-3 (Kif1C) and dynein to coordinate their activity. This adaptor exists within a Fts-Hook3-FHIP1B (FHF) complex, yet how Fts and FHIP1B reinforce the function of Hook3 is poorly understood. How FHF interacts with Kif1C is similarly unclear. To address these questions, we determined the cryo-EM structure of Fts-Hook3-FHIP1B in the absence or presence of the Kif1C tail. FHF adopts one major stable configuration that is characterised by four key binding sites. Within the FHF, Hook3 forms a coiled-coil that breaks and is captured by the alpha-solenoid protein FHIP1B. The Fts E2 variant protein is sandwiched between Hook3 and FHIP1B, contacting the side of the complex and wrapping around the Hook3 coiled-coil cargo adaptor to lock it in place. Together with cross-linking mass spectrometry, we show that Kif1C binds to the third coiled-coil region of Hook3. This density projects towards Fts/FHIP1B nexus without causing a major conformational change in the FHF complex. Collectively, these data reveal the molecular organisation of the FHF complex and the interaction network that enables Hook3 to bind a bidirectional motor complex.