Protein Ubiquitination

Fujita Y, Krause G, Scheffner M et al (2002) Hakai, a c-Cbl-like protein, ubiquitinates and induces endo-cytosis of the E-cadherin complex. Nat Cell Biol 4(3) 222-231... 

Fig. 2.5.5 A study examining the conformational changes of the protein ubiquitin, showing the population ratio of the A-state to the native-state as a function of time, (a) The reaction from 0 to 120 s. (b) The reaction for the first 40 s, including curves fit to a single exponential. Reprinted with permission from Ref. [37]. Copyright (2003) American Chemical Society.

Proteins which are destined for degradation by the proteosome are first modified by the enzyme-catalysed attachment of numerous molecules of the protein ubiquitin, through amino groups to the protein targeted for degradation. This marks out the protein for ATP-dependent hydrolysis by the 26S proteosome, releasing peptides and ubiquitin... 

The proteasome is a cytoplasmic protein degradation machine that hydrolyses proteins, which have been designated for degradation by having a number of molecules of the small ubiquitous protein, ubiquitin, attached to them. 

Fig. 8.2 Ori entations of an amide NH dipolar coupling bond-vector of the protein ubiquitin. Each cone of orientations is compatible with two different alignment directions adopted by the protein in two different alignment media. The central lines defining each cone correspond to the orientations obtained from the measured dipolar couplings. The outer lines include orientations that are possible if the dipolar coupling values are either increased or decreased by 1 Hz. The angle at which the two cones intersect is defined by ft. The solid dot at the cone intersection determines the orientation of the dipolar coupling vector. (Reproduced with permission from B. E. Ramirez and A. Bax, J. Am. Chem. Soc. 1998, 720, 9106-9107.)...

Haas, A. L, Waems, J. V., Heeshko, A., and Rose, 1. A. Ubiquitin-activating enzyme. Mechanism and role in protein-ubiquitin conjugation, J Biol Chem 1982, 257, 2543-2548. 

Fig. 4.1. Fundamentals of the ubiquitin system. Adapted from Ref [5]. Figure 4.1 shows the fundamentals of the ubiquitin system. (1) Ubiquitin is synthesized in linear chains or as the N-terminal fusion with small ribosomal subunits that are cleaved by de-ubiquitylating enzymes to form the active protein. Ubiquitin is then activated in an ATP-dependent manner by El where a thiolester linkage is formed. It is then transthiolated to the active-site cysteine of an E2. E2s interact with E3s and with substrates and mediate either the indirect (in the case of HECT E3s) or direct transfer of ubiquitin to substrate. A number of factors can affect this process. We know that interactions with Hsp70 can facilitate ubiquitylation in specific instances and competition for lysines on substrates with the processes of acetylation and sumoylation may be inhibitory in certain instances. (2) For efficient proteasomal targeting to occur chains of ubiquitin linked internally through K48 must be formed. This appears to involve multiple...

Peng, J., Schwartz, D., Elias, J. E., Thoreen, C. C., Cheng, D., Marsischky, G., Roelofs, J., Einley, D., and Gygi, S. P. Aproteomics approach to understanding protein ubiquitination. Nature Biotechnol. 

ScHEFFNER, M., NuBER, U. and Huibregtse, j. M. Protein ubiquitination involving an El—E2—E3 enzyme ubiquitin thioester cascade. Nature 1995, 373, 81-3. 

Tedesco, D., Lukas, J. and Reed, S. I. The pRb-related protein pl30 is regulated by phosphorylation-dependent proteolysis via the protein-ubiquitin ligase SCF(Skp2). Genes Dev 2002, 16, 2946-57. 

UBPy/USPS human increase in protein ubiquitination cell-growth regulation [119]... 

Figure 2. Protein interaction domains are used by E3 protein-ubiquitin ligases to recruit substrates for ubiquitination, as well as E2 enzymes. Ubiquitination targets contain specific peptide motifs which bind interaction domains in the E3 protein/complex. These interactions can require phosphorylation on tyrosine (pTyr) or threonine (pThr), or the presence of proline-rich motifs (PPXY). The papilloma vims E6/E6AP complex can recognize substrates that contain PDZ domains, which bind a C-terminal motif in the E6 viral protein. See text for details.

Figure 3. The EBV protein LMP2A binds the SH2 domains of the Lyn and Syk B cell tyrosine kinases, and the WW domains of the AIP4 E3 protein-ubiquitin ligase. The protein-protein interactions elicited by the viral protein lead to the ubiquitination and destabilization of Lyn (and potentially other B cell proteins), which may contribute to the inhibition of BCR signaling in latently infected cells.

These examples illustrate the importance of protein-protein interactions in recruiting protein-ubiquitin ligases and their substrates into common complexes that ensure the specificity of substrate recognition. It is evident... 

Winberg, G., Matskova, L., Chen, F., Plant, P., Rotin, D., Gish, G., Ingham, R., Emberg, I., and Pawson, T. (2000). Latent membrane protein 2A of Epstein-Barr virus binds WW domain E3 protein-ubiquitin ligases that ubiquitinate B-cell tyrosine kinases. Mol Cell Biol 20, 8526-35. 

The degradation of protein-ubiquitin conjugates occurs in an ATP-dependent reaction within a large protease complex, the 26S proteasome (review Baumeister et al., 1998). The substrate protein is degraded to peptides in the 26S proteasome, while the ubiquitin is released and again available to form protein conjugates. 

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