Fbx2: Novel mechanisms for glycoprotein destruction and links to hearing and memory

Cellular protein quality control consists primarily of protein folding and degradation pathways. Most proteins destined for degradation are processed by the ubiquitin proteasome pathway, in which E3 ubiquitin ligases recognize and tag specific groups of proteins with ubiquitin chains for degradation by the proteasome. Fbx2 is a F-box E3 ubiquitin ligase subunit that binds high mannose N-linked glycoproteins, which are signature elements of proteins in the endoplasmic reticulum (ER). The goals of this study were to investigate (1) the mechanism by which Fbx2 mediates the handling of glycoproteins and (2) the role of Fbx2 in mammalian organ systems using Fbx2 deficient mice.; Using cell culture systems, primary neurons, and knockout mice, we find that Fbx2 acts in a novel manner to regulate glycoprotein turnover: binding to and functioning with the E3/E4 ubiquitin ligase CHIP (C-terminus of HSC70 interacting protein) in the ubiquitination and degradation of glycoproteins. CHIP binds the N-terminal PEST domain of Fbx2 and likely functions as an E4 ubiquitin chain elongation factor in the turnover of Fbx2-bound neuronal glycoproteins.; We further show that loss of Fbx2 in the mouse has organ-specific phenotypic consequences for the organs in which the gene is expressed (pancreas, brain and cochlea). These range from grossly normal pancreatic function, to cortical synaptic spine loss and associated long-term memory deficits, to age-related hearing loss, alterations in Skp1 protein levels and cellular degeneration in the cochlea.; This study reveals a novel mechanism for glycoprotein degradation through the cooperation of Fbx2 and CHIP, extending the repertoire of pathways by which F-box proteins can regulate ubiquitination. This study also suggests the importance of ubiquitin-dependent pathways in long-term memory, synapse homeostasis, and hearing loss, the most common sensory deficit in humans. Finally, these studies may provide an important clue to the understanding and treatment of age-related hearing loss in humans.