Ubiquitin family proteins and their relationship to the proteasome: a structural perspective.

Many biological processes rely on targeted protein degradation, the dysregulation of which contributes to the pathogenesis of various diseases. Ubiquitin plays a well-established role in this process, in which the covalent attachment of polyubiquitin chains to protein substrates culminates in their degradation via the proteasome. The three-dimensional structural topology of ubiquitin is highly conserved as a domain found in a variety of proteins of diverse biological function. Some of these so-called "ubiquitin family proteins" have recently been shown to bind components of the 26S proteasome via their ubiquitin-like domains, thus implicating proteasome activity in pathways other than protein degradation. In this chapter, we provide a structural perspective of how the ubiquitin family of proteins interacts with the proteasome.