Mono-ubiquitin

Lai, Z., et al., Human mdm2 mediates multiple mono-ubiquitination of p53 by a mechanism requiring enzyme isomerization. J Biol Chem, 2001, 276(33), 31357-67. 

Ubiquitin-protein ligases promote not only the attachment of ubiquitin to the protein substrates but also the extension of the ubiquitin chain. What determines the choice between mono- vs. polyubiquitination is not well understood. It is possible that certain E3s catalyze only mono-ubiquitination. Alternatively, factors other than E3s might be responsible for the attachment of a single ubiquitin. Eor example, ubiquitin-binding accessory proteins have been suggested to block extension of the ubiquitin chain [25], whereas E3-associated ubiquitin hydrolase could trim down the polyubiquitin chain. The identification and characterization of ubiquitin E2 variant proteins (UEVs) have provided an explanation for the assembly of K63-linked polyubiquitin chains [26, 27]. As discussed later, UEVs can be considered as special E3s, with the ubiquitin chain as their substrates. 

Haglund, K. et al. Multiple mono-ubiquitination of RTKs is sufficient for their endocytosis and degradation. Nat Cell Biol 2003, 5, 461-6. 

Salvage Pathways Recovering Mono-ubiquitin Adducts and Recycling Polyubiquitin... 

Regulating mono-ubiquitination of proteins by DUBs is important in histone modification where ubiquitination is thought to modulate chromatin structure and transcriptional activity. Normally, about 10% of the histone core octomers contain ubiquitinated histones and the ubiquitin is removed at mitosis by DUB activity. UBP8 has been demonstrated to regulate the ubiquitination of histone H2B, which is important in transcriptional activation of many genes [88]. 

One fascinating observation is that PCNA (proliferating cell nuclear antigen) can be modified by multiple forms of ubiquitin, demonstrating that DUBs with different specificities can act at the same location on a specific substrate. PCNA can be modified by mono-ubiquitin, 63-linked polyubiquitin, or SUMO at K164 [89]. Modification of PCNA by mono- or polyubiquitin determines whether it is utilized in translesion synthesis or error-free DNA repair, respectively. SUMO modification prevents PCNA function in DNA repair and instead promotes DNA replication. It is probable that multiple DUBs, as yet unidentified, are required to regulate PCNA modification. 

This labeling has been used with success in yeast where 6 of the 17 known DUBs were labeled with UbVS [115]. Incomplete labeling likely results from DUBs that do not act on mono-ubiquitin or where the UbVS could not access the active site. The labeling has also been used with great success in mammalian cell lysates to identify novel ubiquitin DUBs [41]. A novel deneddylating enzyme and a novel DUB that acts on autophagy-related UbL proteins have also been identified using vinyl sulfone labeled probes [26, 37, 116]. 

As will be discussed in more detail elsewhere in this book, ubiquitin attachment to proteins gives rise to a number of different signals. Although it was initially thought to be required only for proteasomal degradation, we now know that there are other ubiquitin-based signals, such as mono-ubiquitination, or... 

Panlp- Panlp in yeast, and its mammalian homologue, EGFR protein substrate 15 (EpslS), are essential for normal endocytosis (Carbone et al. 1997 Benmerah et al. 1998 Wendland and Emr 1998). Although these proteins are associated with clathrin complexes and genetic evidence raised the possibility that Panlp may act as an adaptor connecting RspSp to potential ubiquitination substrates, physical associations between RspSp and Panlp have not been detected. In mammalian cells, EpslS is tyrosine-phosphorylated and mono-ubiquitinated upon EGF stimulation (van Delft et al. 1997). The tyrosine-phosphorylation of the protein may play a role in... 

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