Degradation in the Proteasome

The 26S proteasome is composed of two protein aggregates, a 19S and a 20S particle. The main proteolytic component of the 26S proteasome is the 20S particle, the structure of which from an archaebacterial system and a yeast system has been solved (Lowe et ah, 1995 Groll et ah, 1997). 

The structure of the 20S proteasome (Fig. 2.13) from Thermoplasma acidophilum displays four rings stacked upon each other surrounding a central cavity in which proteolysis takes place. An N-terminal threonine has been identified as an essential active site residue of the protease center. The OH-group of the threonine functions as a nucleophile during hydrolysis of the petide bond. A similar mechanism of hydrolysis has been shown for other hydrolases, which, because of this property, are now included in the family of N-terminal nucleophile hydrolases. For some /1-subunits of eucaryotes the N-terminal threonine is generated by autoproteolysis of an N-terminal prosequence. 

The presence of the protease center in the central cavity ensures that the proteolysis is compartmentalized and shielded from the surrounding media. The substrate proteins are accessible to the proteolytic center only via a ring-shaped opening at the end of the 20S proteasome and require the assistance of the 19S particle. The structure of the 20S proteasome also indicates that proteins are accessible to the catalytic center only in the unfolded state. 

A distinct feature of the vertebrate proteasome is the possibility to exchange specific subunits thereby generating proteasome variants. By exchange of /1-subunits different 

20S complexes can form which function in specific degradation reactions. In the pro-teasome that is involved in processing antigenic peptides the catalytic reactions are performed by E-subunits different from the the X,Y and Z subunits. This specific proteasome is also called immunoproteasome . 

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Protein processing in the endoplasmic reticulum makes mistakes. All membrane-associated proteins and proteins that are secreted by the cell are synthesised on membrane-bound ribosomes and pass into the lumen of the reticulum, where they are modified by post-translational processes, so that much biochemical manipulation of the proteins takes place. Consequentially mistakes are often made. Such abnormal proteins are exported from the lumen into the cytosol for ubiquitination and degradation in the proteasome. 

Proteins destined for degradation in the proteasome (e.g., incorrectly folded or old... 

Arrest in Gx phase can now be achieved in at least two ways, depending on the substrates of the Chkl and Chk2 enzymes. In one rapid way, the dual specificity phosphatase Cdc25C is phosphorylated on Ser 123 and is thereby targeted for ubiquitina-tion and degradation in the proteasome pathway. The lack of this enzyme locks the CDK2 kinase in the inactive form phosphorylated on threonine 14 and tyrosine 15.The cyclin E-CDK2 complex that is required for entry into S phase is inhibited, and the cell cycle arrests at Gj/S. It should be noted that the scheme in Fig. 13.19 is only a minimal scheme that does not address the participation of numerous other proteins that function as adaptors or structural proteins in these processes. 

ER-Associated Degradation, when proteins mis-fold in the ER due to mutation or environmental conditions, they are selectively exported to the cytosol for degradation by the proteasome. 

The UPS also plays a major role in protein quality control. In a process known as endoplasmic associated degradation (ERAD), misfolded proteins, which are formed in the endoplasmatic reticulum, are translocated back to the cytoplasm and degraded by the proteasome. 

Mehle A, Strack B, Ancuta P, Zhang C, McPike M, Gabuzda D (2004) Vif overcomes the innate antiviral activity of APOBEC3G by promoting its degradation in the ubiquitin-proteasome pathway. J Biol Chem 279(9) 7792-7798... 

Amerik, A., Swaminathan, S., Krantz, B. a., Wilkinson, K. D., and Hochstrasser, M. In vivo disassembly of free polyubiquitin chains by yeast Ubpl4 modulates rates of protein degradation by the proteasome, EmboJ, 1997, 16, 4826-38. 

The N-terminal extensions are removed thereby generating a new unblocked N-terminal threonine in the catalytically active yS-subunits. A small accessory protein called Umpl in yeast or proteassemblin in mammalian cells assists in the final assembly of the 20S proteasome [132], Interestingly Umpl/POMP is apparently trapped in the proteasome s central chamber and degraded upon maturation of the enzyme [133]. 

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