Coordination between actin and microtubule motors during endocytosis

The cell’s ability to internalise materials for nutrient acquisition, molecule recycling and pathogen clearance is essential for life. Termed endocytosis, this process depends on a dynamic network of interactions between the cytoskeleton and its molecular motors. Actin filaments and myosin motors invaginate the plasma membrane around ingested material for vesicle formation and short-range transport, while the microtubule-based motors dynein and kinesin drive long-range endosome movement towards degradative or recycling pathways. The fate of the endosome relies on a seamless transition between actin- and microtubule-based movement. Our lab aims to uncover the unknown mechanisms behind this critical switch. Specifically, we want to understand how actin and microtubule motors are regulated and recruited to early endosomes, and how they coordinate to successfully transition across cytoskeletal tracks. Addressing these questions is key to not only understanding how a cell organises its contents but also how endocytic processes are hijacked (pathogenesis) or dysregulated (neurodegenerative diseases, e.g., Alzheimer’s).

During your PhD, you will dissect the role of endosome-bound regulators in motor recruitment, focusing on the Fts-HOOK-FHIP (FHF) adaptor family. These adaptors form through specific combinations of HOOKs and FHIPs to link various motors to organelle membranes. For example, Fts-HOOK2 binds FHIP2A for ER-to-Golgi transport, while Fts-HOOK3 binds FHIP1B for early endosome sorting. The specificity of these interactions and why certain motors are recruited to distinct FHF subsets is unknown. You will use cryo-EM to compare FHF complexes to understand the molecular basis for their specific assembly. To explore how endosomes transition across cytoskeletal tracks, you will focus on Fts-HOOK3-FHIP1B as a prime candidate. This complex binds proteins on the early endosome and dynein, multiple kinesins and myosins. You will ask whether FHF couples actin and microtubule-based motors or competes for their binding. To this end, you will reconstitute FHF-motor interactions and map binding sites using cryo-EM and crosslinking mass-spectrometry. Live fluorescence imaging of FHF-bound motor co-complexes on vesicles will reveal the types and numbers of motors sufficient to switch endosomes at actin-microtubule intersections. The knowledge gained here will be used to perturb FHF interactions in cells and analyse how endocytosis is affected. Collectively, these projects will establish the molecular mechanism of this cargo adaptor family and its role in the progression of the early endosome. They will also provide you with a highly sought broad-based training in structural biology and molecular cell biology, enabling you to take the next steps in academic research or industry.