Structural highlights
Function
UBC_HUMAN Ubiquitin exists either covalently attached to another protein, or free (unanchored). When covalently bound, it is conjugated to target proteins via an isopeptide bond either as a monomer (monoubiquitin), a polymer linked via different Lys residues of the ubiquitin (polyubiquitin chains) or a linear polymer linked via the initiator Met of the ubiquitin (linear polyubiquitin chains). Polyubiquitin chains, when attached to a target protein, have different functions depending on the Lys residue of the ubiquitin that is linked: Lys-6-linked may be involved in DNA repair; Lys-11-linked is involved in ERAD (endoplasmic reticulum-associated degradation) and in cell-cycle regulation; Lys-29-linked is involved in lysosomal degradation; Lys-33-linked is involved in kinase modification; Lys-48-linked is involved in protein degradation via the proteasome; Lys-63-linked is involved in endocytosis, DNA-damage responses as well as in signaling processes leading to activation of the transcription factor NF-kappa-B. Linear polymer chains formed via attachment by the initiator Met lead to cell signaling. Ubiquitin is usually conjugated to Lys residues of target proteins, however, in rare cases, conjugation to Cys or Ser residues has been observed. When polyubiquitin is free (unanchored-polyubiquitin), it also has distinct roles, such as in activation of protein kinases, and in signaling.[1] [2]
Publication Abstract from PubMed
NBR1 (neighbor of BRCA1 gene 1) is a protein commonly found in ubiquitin-positive inclusions in neurodegenerative diseases. Due to its high architectural similarity to the well-studied autophagy receptor protein p62/SQSTM1, NBR1 has been thought to analogously bind to ubiquitin-marked autophagic substrates via its carboxy-terminal ubiquitin-associated (UBA) domain and deliver them to autophagosomes for degradation. Unexpectedly, we find that NBR1 differs from p62 in its UBA structure and accordingly in its interaction with ubiquitin. Structural differences are observed on helix alpha-3, which is tilted further from helix alpha-2 and extended by approximately one turn in NBR1. This results not only in inhibition of a p62-type self-dimerization of NBR1 UBA, but also in a significantly higher affinity for monoubiquitin as compared with p62 UBA. Importantly, the NBR1 UBA-ubiquitin complex structure shows that the negative charge of the side chain in front of the conserved MGF motif in the UBA plays an integral role in the recognition of ubiquitin. In addition, NMR and ITC experiments show that NBR1 UBA binds to each monomeric unit of polyubiquitin with similar affinity and by the same surface used for binding to monoubiquitin. This indicates that NBR1 lacks polyubiquitin linkage-type specificity, in good agreement with the non-specific linkages observed in intracellular ubiquitin-positive inclusions. Consequently, our results demonstrate that the structural differences between NBR1 UBA and p62 UBA result in a much higher affinity of NBR1 for ubiquitin, which in turn suggests that NBR1 may form intracellular inclusions with ubiquitylated autophagic substrates more efficiently than p62.
Solution Structure of the Ubiquitin-associated (UBA) Domain of Human Autophagy Receptor NBR1 and its Interaction with Ubiquitin and Polyubiquitin.,Walinda E, Morimoto D, Sugase K, Konuma T, Tochio H, Shirakawa M J Biol Chem. 2014 Apr 1. PMID:24692539[3]
From MEDLINE®/PubMed®, a database of the U.S. National Library of Medicine.
References
- ↑ Huang F, Kirkpatrick D, Jiang X, Gygi S, Sorkin A. Differential regulation of EGF receptor internalization and degradation by multiubiquitination within the kinase domain. Mol Cell. 2006 Mar 17;21(6):737-48. PMID:16543144 doi:S1097-2765(06)00120-1
- ↑ Komander D. The emerging complexity of protein ubiquitination. Biochem Soc Trans. 2009 Oct;37(Pt 5):937-53. doi: 10.1042/BST0370937. PMID:19754430 doi:10.1042/BST0370937
- ↑ Walinda E, Morimoto D, Sugase K, Konuma T, Tochio H, Shirakawa M. Solution Structure of the Ubiquitin-associated (UBA) Domain of Human Autophagy Receptor NBR1 and its Interaction with Ubiquitin and Polyubiquitin. J Biol Chem. 2014 Apr 1. PMID:24692539 doi:http://dx.doi.org/10.1074/jbc.M114.555441
