Structure of the Ubiquitin-associated Domain of p62 (SQSTM1) and Implications for Mutations That Cause Paget's Disease of Bone

The p62 protein (also known as SQSTM1) mediates diverse cellular functions including control of NFkappaB signaling and transcriptional activation. p62 binds non-covalently to ubiquitin and co-localizes with ubiquitylated inclusions in a number of human protein aggregation diseases. Mutations in the gene encoding p62 cause Paget's disease of bone (PDB), a common disorder of the elderly characterized by excessive bone resorption and formation. All of the p62 PDB mutations identified to date cluster within the C-terminal region of the protein, which shows low sequence identity to previously characterized ubiquitin-associated (UBA) domains. We report the first NMR structure of a recombinant polypeptide that contains the C-terminal UBA domain of the human p62 protein (residues 387-436). This sequence, which confers multiubiquitin chain binding, forms a compact three-helix bundle with a structure analogous to the UBA domains of HHR23A but with differences in the loop regions connecting helices that may be involved in binding accessory proteins. We show that the Pro392 --> Leu PDB substitution mutation modifies the structure of the UBA domain by extending the N terminus of helix 1. In contrast to the p62 PDB deletion mutations that remove the UBA domain and ablate multiubiquitin chain binding, the Pro392 --> Leu substitution does not affect interaction of the UBA domain with multiubiquitin chains. Thus, phenotypically identical substitution and deletion mutations do not appear to predispose to PDB through a mechanism dependent on a common loss of ubiquitin chain binding by p62.