Ubiquitin (Ub) is a highly conserved 76 amino-acid protein found throughout eukaryotic cells. A vast number of cellular processes, including targeted protein degradation, cell cycle progression, DNA repair, protein trafficking, inflammatory response, virus budding, and receptor endocytosis, are regulated by Ub-mediated signalling; where the target protein is tagged by single or multi-monomeric Ub (monomeric Ub attached to multiple sites on the substrate) or a polymeric chain of Ubs (Fushman and Walker, 2010). More recently the demonstration that ubiquitin itself can be modified through phosphorylation by the kinase PTEN Induced putative Kinase1 (PINK1) provides a major breakthrough linking the two most important signalling pathways in cells; phosphorylation and ubiquitylation (Kane et al., 2014; Kazlauskaite et al., 2014; Koyano et al., 2014). Several studies have revealed that PINK1 directly phosphorylates ubiquitin on Ser65 a residue that is also shared by the Parkin Ubl domain (Kane et al., 2014; Kazlauskaite et al., 2014; Koyano et al., 2014). Parkin is activated by Ser65 phosphorylated ubiquitin in a manner which is independent of ubiquitin's ability to be conjugated to lysine residues on target proteins. The mechanism of Parkin priming and activation is thought to occur through a conformational change induced by PINK1 phosphorylation on Ser65 followed by the binding of PINK1 Ser65 phosphorylated ubiquitin on the RING1 domain which optimises the ubiquitylation activity of Parkin (Kazlauskaite et al., 2014; Koyano et al., 2014). Phospho-ubiquitin may play other roles in regulating Parkin but more generally the identification of phospho-ubiquitin as a second messenger in signalling pathways could reveal the existence of further ubiquitin phosphatases and lead to the discovery of additional kinase and ubiquitin related substrates (Sauve and Gehring, 2014).
Phosphoproteomic studies have identified the presence of several phosphorylated peptides demonstrating homology to proteins of the Ubiquitin Proteasome Pathway (UPP) these include ubiquitin (pSer57 being among those identified), ubiquitin like modifiers and proteins containing ubiquitin binding domains (Bian et al., 2014; Bennetzen et al., 2010; Kettenbach et al., 2011; Sharma et al., 2014).
Thus biotinylated versions of phosphorylated ubiquitin that have been identified in such phosphoproteomic studies provide tools for probing the possible roles and functions of these species in signalling pathways (eg in pull down/capture assays).
Biotin-Ahx-ubiquitin (pSer57) (Cat# 60-0205-050) is a phosphorylated ubiquitin which may be used in experiments alongside the non-phosphorylated control Biotin-Ahx-ubiquitin (synthetic) (Cat# 60-0201-050).
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