NORMAL GENOMIC

• Bartee E, Mansouri M, Hovey Nerenberg BT, Gouveia K, Fruh K
• Downregulation of major histocompatibility complex class I by humanubiquitin ligases related to viral immune evasion proteins.
• J Virol. 2004; 78: 1109-20
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• Poxviruses and gamma-2 herpesviruses share the K3 family of viral immuneevasion proteins that inhibit the surface expression of glycoproteins suchas major histocompatibility complex class I (MHC-I), B7.2, ICAM-1, andCD95(Fas). K3 family proteins contain an amino-terminal PHD/LAP or RING-CHdomain followed by two transmembrane domains. To examine whether humanhomologues are functionally related to the viral immunoevasins, we studiedseven membrane-associated RING-CH (MARCH) proteins. All MARCH proteinslocated to subcellular membranes, and several MARCH proteins reducedsurface levels of known substrates of the viral K3 family. Two closelyrelated proteins, MARCH-IV and MARCH-IX, reduced surface expression ofMHC-I molecules. In the presence of MARCH-IV or MARCH-IX, MHC-I wasubiquitinated and rapidly internalized by endocytosis, whereas MHC-Imolecules lacking lysines in their cytoplasmic tail were resistant todownregulation. The amino-terminal regions containing the RING-CH domainof several MARCH proteins examined catalyzed multiubiquitin formation invitro, suggesting that MARCH proteins are ubiquitin ligases. Thefunctional similarity of the MARCH family and the K3 family suggests thatthe viral immune evasion proteins were derived from MARCH proteins, anovel family of transmembrane ubiquitin ligases that seems to targetglycoproteins for lysosomal destruction via ubiquitination of thecytoplasmic tail.

• Werner ED, Brodsky JL, McCracken AA
• Proteasome-dependent endoplasmic reticulum-associated protein degradation:an unconventional route to a familiar fate.
• Proc Natl Acad Sci U S A. 1996; 93: 13797-801
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• Until recently, the degradation of aberrant and unassembled proteinsretained in the endoplasmic reticulum (ER) was thought to involveunidentified ER-localized proteases. We now show that the ER-associateddegradation (ERAD) of two mutant proteins that accumulate in the ER lumenis inhibited in a proteasome-defective yeast strain and when cytosol fromthis mutant is used in an in vitro assay. In addition, ERAD is limited invitro in the presence of the proteasome inhibitors,3,4-dichloroisocoumarin and lactacystin. Furthermore, we find that an ERADsubstrate is exported from ER-derived microsomes, and the accumulation ofexported substrate is 2-fold greater when proteasome mutant cytosol isused in place of wild-type cytosol. We conclude that lumenal ERADsubstrates are exported from the yeast ER to the cytoplasm for degradationby the proteasome complex.

• Finger A, Knop M, Wolf DH
• Analysis of two mutated vacuolar proteins reveals a degradation pathway inthe endoplasmic reticulum or a related compartment of yeast.
• Eur J Biochem. 1993; 218: 565-74
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• The fate of a mutant form of each of the two yeast vacuolar enzymesproteinase yscA (PrA) and carboxypeptidase yscY (CPY) has beeninvestigated. Both mutant proteins are rapidly degraded after entering thesecretory pathway. Mutant PrA is deleted in 37 amino acids spanning theprocessing site region of the PrA pro-peptide. The mutant enzyme shows noactivity towards maturation of itself or other vacuolar hydrolases, afunction of wild-type PrA. Mutant CPY carries an Arg instead of a Glyresidue in a highly conserved region, two positions distant from theactive-site Ser. In contrast to wild-type CPY, the mutant form was quicklydegraded by trypsin in vitro, indicating an altered structure. Usingantisera specific for alpha-1-->6 and alpha-1-->3 outer-chain mannoselinkages, no Golgi-specific carbohydrate modification could be detected oneither mutant protein. Subcellular fractionation studies located bothmutant enzymes in the endoplasmic reticulum. Degradation kinetics of bothproteins show the same characteristics, indicating similar degradationpathways. The degradation process was shown to be independent of afunctional sec18 gene product and takes place before Golgi-specificcarbohydrate modifications occur. The proteasome, the major proteolyticactivity of the cytoplasm, is not involved in this degradation event. Alldegradation characteristics of the two mutant proteins are consistent witha degradation process within the endoplasmic reticulum ('ER degradation').

• David LL, Shearer TR
• Role of proteolysis in lenses: a review.
• Lens Eye Toxic Res. 1989; 6: 725-47
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• It has been suggested that proteases are involved in removal of damaged orobsolete proteins and/or that the activation of proteases could contributeto cataract formation. This review summarizes the properties of severalrecently studied lens endopeptidases including: trypsin-like protease,multicatalytic endopeptidase complex, membrane bound proteases, andcalpain. Properties discussed include composition, substrate specificity,distribution, changes in activity during aging, and regulation.Additionally, properties of the lens ubiquitin conjugation system arereviewed. When possible, an attempt was made to relate these findings towhether the lens proteolytic activity was involved in clearing damagedproteins, or whether it could contribute to cataract formation. Clearingof damaged or obsolete lens proteins may involve the participation ofseveral protease activities. Findings suggest that lens proteaseactivities are lost at variable rates during aging, and differ inconcentration between species. It was concluded that the consequence ofproteolytic activity in the lens may depend closely on the compliment ofproteolytic activities found. For instance, proteases causing only partialdegradation of lens proteins may predominate in lenses undergoing cataractformation, while proteases assisting in the removal of partially degradedproteins are lost. The partially degraded lens proteins, as well as otherdenatured lens proteins, may then accumulate and lead to cataractformation.