Enabling Protein Degradation Drug Discovery

Deconjugating enzymes (DCEs) are proteases that process ubiquitin or ubiquitin-like gene products, reverse the modification of proteins by a single ubiquitin or ubiquitin-like protein (UBL) and remodel polyubiquitin (or poly-UBL) chains on target proteins (Reyes-Turcu, et al., 2009). The deubiquitylating – or deubiquitinating – enzymes (DUBs) represent the largest family of DCEs and regulate ubiquitin-dependent signalling pathways. The activities of the DUBs include the generation of free ubiquitin from precursor molecules, the recycling of ubiquitin following substrate degradation to maintain cellular ubiquitin homeostasis and the removal of ubiquitin or ubiquitin-like proteins (UBL) modifications through chain editing to rescue proteins from proteasomal degradation or to influence cell signalling events (Komander, et al., 2009). There are two main classes of DUB, cysteine proteases and metalloproteases. AMSH is a member of the JAB1/MPN/Mov34 metalloenzyme (JAMM) family and cloning of the human gene was first described by Tanaka et al. (1999). AMSH and AMSH-LP share 54% identity and 75% sequence similarity in their JAMM domain. It is known that both proteins act as regulators of free ubiquitin in the cell, bind clathrin, and contain a putative nuclear localization signal and a microtubule interacting and transport (MIT) domain. AMSH also contains a Src homology 3 (SH3) domain, facilitating its interaction with signal transducing adaptor molecule (STAM), while this functional SH3-binding motif is lost in AMSH-LP (Davies, et al., 2011). STAM complexed with hepatocyte growth factor regulated tyrosine kinase substrate (Hrs) organises cargo proteins in the multivesicular body (MVB) pathway. AMSH is thought to play a role in regulation of ubiquitin-mediated degradation by binding to STAM (Kim, et al., 2006). AMSH also plays a significant role in neurodegeneration, demonstrated by AMSH knockout mice exhibiting severe neuronal damage, specifically neuron loss and increasing numbers of apoptotic cells (Suzuki, et al., 2011). AMSH is well known to specifically cleave K63-linked polyubiquitin chains and does not cleave K48-linked polyubiquitin chains (Sato, et al., 2008). After removal of these K63-linked polyubiquitin chains, AMSH can coordinate the recycling of receptors to the cell surface (McCullough, et al., 2004).