Role of the Diaphanous Formin FHOD1 and its interaction with nesprin-2-giant in nuclear migration

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FHOD1 is an atypical member of the Diaphanous Related Formin (DRF) protein family that does not nucleate actin polymerization but bundles actin filaments, decorates cellular actin structures, and coordinates actin with microtubule cytoskeletons. Our previous work identified FHOD1 as component of the machinery governing nuclear movement of migrating fibroblasts that is essential for actin-mediated positioning of the nucleus as well as for centrosome reorientation. To facilitate nuclear positioning, FHOD1 associates with actin cables via an N-terminal actin binding site and interacts with the outer nuclear envelope protein nesprin-2-giant (N2G). Having established this basic principle of how FHOD1 facilitates actin dependent nuclear positioning, the first goal of this proposal is to gain insight into this unusual mechanism at the structural and functional level. To this end we will define precisely the surfaces involved in the interactions of FHOD1 with N2G and actin, characterize their biochemical and functional properties, and investigate regulatory mechanisms. This will include the generation of X-ray structures of complexes between the interacting domains of two binding partners with the final goal of solving the structure of the tripartite complex of all three interacting domains. These analyses should also allow us to define the molecular signatures that determine its atypical mode of action among DRF proteins. The second main goal of this project is to elucidate how FHOD1-N2G interactions are regulated and which additional FHOD1 binding partners are involved in FHOD1 function. Our preliminary results identified the kinesin-2 protein KIF3C as novel FHOD1 binding partner essential for nuclear migration. Mechanistic analyses will now focus on dissecting the mechanisms by which FHOD1 coordinates actin and microtubule cytoskeletons in nuclear migration with particular focus on the role of KIF3C and other newly identified FHOD1 ligands. Together, this interdisciplinary approach combining the expertise in cell biology and structural biology/biochemistry of the Fackler and Geyer laboratories will yield important new insights into the molecular mechanism used by the atypical DRF FHOD1 for the dynamic transport of the largest cellular organelle.