Brain-Derived 11S Regulator (PA28αß) Promotes Proteasomal Hydrolysis of Elongated Oligoglutamine-Containing Peptides.

Proteins with extended polyglutamine regions are associated with several neurodegenerative disorders, including Huntington's disease. Intracellular proteolytic processing of these proteins is not well understood. In particular, it is unclear whether long polyglutamine fragments resulting from the proteolysis of these proteins can be potentially cleaved by the proteasome. Here, we studied the susceptibility of the glutamine-glutamine bond to proteolysis by the proteasome using oligoglutamine-containing peptides with a fluorophore/quencher pair. We found that the addition of the 11S proteasomal regulator (also known as PA28) significantly accelerated the hydrolysis of oligoglutamine-containing peptides by the 20S proteasome. Unexpectedly, a similar effect was observed for the 26S proteasome in the presence of the 11S regulator. LC/MS data revealed that the hydrolysis of our peptides with both 20S and 26S proteasomes leads to N-terminal fragments containing two or three glutamine residues and that the hydrolysis site does not change after the addition of the 11S regulator. This was confirmed by the docking experiment, which shows that the preferred hydrolysis site is located after the second/third glutamine residue. Inhibitory analysis revealed that trypsin-like specificity is mainly responsible for the proteasomal hydrolysis of the glutamine-glutamine bond. Together, our results indicate that both 20S and 26S proteasomes are capable of degrading the N-terminal part of oligoglutamine fragments, while the 11S regulator significantly accelerates the hydrolysis without changing its specificity. This data suggests that proteasome activity may be enhanced in relation to polyglutamine substrates present in neurons in the early stages of polyglutamine disorders.

Control of Genome through Variative Nature of Histone-Modifying Ubiquitin Ligases.

Covalent attachment of ubiquitin residue is not only the proteasomal degradation signal, but also a widespread posttranslational modification of cellular proteins in eukaryotes. One of the most important targets of the regulatory ubiquitination are histones. Localization of ubiquitin residue in different regions of the nucleosome attracts a strictly determined set of cellular factors with varied functionality. Depending on the type of histone and the particular lysine residue undergoing modification, histone ubiquitination can lead both to transcription activation and to gene repression, as well as contribute to DNA repair via different mechanisms. An extremely interesting feature of the family of RING E3 ubiquitin ligases catalyzing histone ubiquitination is the striking structural diversity of the domains providing high specificity of modification very similar initial targets. It is obvious that further elucidation of peculiarities of the ubiquitination system involved in histone modification, as well as understanding of physiological role of this process in the maintenance of homeostasis of both single cells and the entire organism, will substantially expand the possibilities of treating a number of socially significant diseases.

Ubiquitin-independent proteosomal degradation of myelin basic protein contributes to development of neurodegenerative autoimmunity.

Recent findings indicate that the ubiquitin-proteasome system is involved in the pathogenesis of cancer as well as autoimmune and several neurodegenerative diseases, and is thus a target for novel therapeutics. One disease that is related to aberrant protein degradation is multiple sclerosis, an autoimmune disorder involving the processing and presentation of myelin autoantigens that leads to the destruction of axons. Here, we show that brain-derived proteasomes from SJL mice with experimental autoimmune encephalomyelitis (EAE) in an ubiquitin-independent manner generate significantly increased amounts of myelin basic protein peptides that induces cytotoxic lymphocytes to target mature oligodendrocytes ex vivo. Ten times enhanced release of immunogenic peptides by cerebral proteasomes from EAE-SJL mice is caused by a dramatic shift in the balance between constitutive and ß1i(high) immunoproteasomes in the CNS of SJL mice with EAE. We found that during EAE, ß1i is increased in resident CNS cells, whereas ß5i is imported by infiltrating lymphocytes through the blood-brain barrier. Peptidyl epoxyketone specifically inhibits brain-derived ß1i(high) immunoproteasomes in vitro (kobs/[I] = 240 M(-1)s(-1)), and at a dose of 0.5 mg/kg, it ameliorates ongoing EAE in vivo. Therefore, our findings provide novel insights into myelin metabolism in pathophysiologic conditions and reveal that the ß1i subunit of the immunoproteasome is a potential target to treat autoimmune neurologic diseases.