A role for
ubiquitin in the pathogenesis of human diseases was first suggested some two decades ago, from studies that localized the
protein to intracellular
protein aggregates, which are a feature of the major human
neurodegenerative disorders. Although several different mechanisms have been proposed to connect impairment of the UPS (
ubiquitin-
proteasome system) to the presence of these '
ubiquitin inclusions' within diseased neurones, their significance in the disease process remains to be fully clarified.
Ubiquitin inclusions also contain
ubiquitin-
binding proteins, such as the p62
protein [also known as SQSTM1 (sequestosome 1)], which non-covalently interacts with the ubiquitinated
protein aggregates and may serve to mediate their autophagic clearance. p62 is a multifunctional
protein and, in the context of bone-resorbing osteoclasts, is an important scaffold in the RANK [
receptor activator of NF-kappaB (
nuclear factor kappaB)]-
NF-kappaB signalling pathway. Further, mutations affecting the UBA domain (
ubiquitin-associated domain) of p62 are commonly found in patients with the skeletal disorder PDB (
Paget's disease of bone). These mutations impair the ability of p62 to bind to
ubiquitin and result in disordered osteoclast
NF-kappaB signalling that may underlie the disease aetiology. Recent structural insights into the unusual mechanism of
ubiquitin recognition by the p62 UBA domain have helped rationalize the mechanisms by which different PDB mutations exert their negative effects on
ubiquitin binding by p62, as well as providing an indication of the
ubiquitin-binding selectivity of p62 and, by extension, its normal
biological functions.