| Summary: | NBRl (Neighbour of BRCAJ) is a ubiquitously expressed scaffold protein that mediates intracellular functions through protein-protein interactions. The C-terminal UBA domain of Nbrl binds mono- and polyubiquitinated chains whilst the N-terminal PB1 domain interacts with the structurally similar protein p62. The aims of this thesis were to further characterise the biological functions of Nbrl by identifying novel protein interactors. A yeast-2-hybrid screen revealed a direct interaction between Nbrl and the autophagic effector protein LC3. This suggested a role for Nbrl in the autophagic degradation of polyubiquitinated proteins. A further yeast-2-hybrid screen identified a direct interaction between Nbrl and the microtubule associated protein MAP IB. It is hypothesised that via its interaction with MAP IB and LC3, Nbrl targets ubiquitinated proteins to the microtubule network where they can be transported to sites of autophagosomal formation. Further putative interactors were identified for Nbrl including the ubiquitin conjugating enzyme, UBE20 and the eukaryotic translation elongation factor eEFl A. These suggested a role for Nbrl as a scaffold to facilitate protein ubiquitination and degradation by the proteasome as eEFl A is involved in the cotranslational degradation of polyubiquitinated proteins. -- In bone, Nbrl was previously identified as a key regulator of osteoblast differentiation and activity. The work described in this thesis strengthened these observations and highlighted that the interaction between Nbrl and p62 is also important for the maintenance of bone. Mice with the NbrlD50R point mutation which inhibits the interaction between Nbrl and p62 displayed an early osteoporotic phenotype due to altered osteoblast function and osteoclast size and nucleation that was subsequently resolved with age.
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