Displacement of housekeeping proteasome subunits by MHC-encoded LMPs: a newly discovered mechanism for modulating the multicatalytic proteinase complex.

Abstract

The degradation of cytoplasmic antigens to peptides presented by class I MHC molecules is thought to be mediated by the ubiquitin/proteasome pathway. Support for this view came from our observation that the subunit composition of proteasomes can be changed by interferon‐gamma (IFN‐gamma) treatment. Thereby two subunits, LMP2 and LMP7, which are encoded in the MHC class II region, are incorporated into the proteasomal complex, whereas other subunits disappear. In the experiments reported in this communication we studied the subunit changes occurring in cell lines where the expression of LMP2 or LMP7 can be regulated individually either by IFN‐gamma induction or by applying a new system to control the expression of transfected LMPs. In both situations LMP2 induction leads exclusively to the disappearance of housekeeping subunit 2, whereas LMP7 affects only subunit 10. Subunit 2 was found to be 76% homologous to LMP2. Since incorporation of LMP2 into the proteasomal complex prevents processing of the subunit 2 precursor, we conclude that LMP2 displaces subunit 2 during assembly. Subunit displacement is most likely a general mechanism to modulate the catalytic activity of the proteasomal complex without changing its structure. Furthermore, the controlled incorporation of transfected subunits into the complex offers a new approach to study proteasome function in vivo.

The proteasome and its role in the degradation of oxidized proteins

“…c-Interferon secreted in inflammatory reactions by CD4 1 and CD8 1 T-cell induces the expression of the so-called inducible forms of b1i, b2i, and b5i replacing the constitutive subunits during de novo synthesis (68), whereas the constitutive forms of b1, b2, and b5 are downregulated (69,70). Since this is a very quick process, many intermediate forms can be observed (71), showing large variation in fragment length produced by the cell during protein degradation.…”

Heat Shock Up-Regulates lmp2 and lmp7 and Enhances Presentation of Immunoproteasome-Dependent Epitopes

“…Only three of the ␤ subunits (two copies of each) are proteolytically active: ␦ (␤1), X (␤5), and Z (␤5). In cells stimulated by the proinflammatory cytokine IFN-␥, the composition of the proteasome is altered such that the three active ␤ subunits are replaced by inducible subunits: lmp2 (␤1i), lmp7 (␤5i), and mecl1 (␤2i) (12)(13)(14)(15)(16)(17). This modified proteasome is the immunoproteasome (18).…”

“…The immunoproteasome is more likely to generate peptides with hydrophobic and basic C-terminal residues and less likely to generate peptides with acidic C-terminal residues (17,19,20). A number of antigenic epitopes are differentially processed by immunoproteasome-expressing cells.…”

In vivo manipulation of interleukin-2 expression by a retroviral tetracycline (tet)-regulated system

“…Expression of a target gene inserted downstream of the tetO/minimal CMV promoter is dependent upon the tTA, which binds tetO sequences. In the presence of the antibiotic, the binding of the tTA to the tetO promoter is impaired, resulting in shutoff of target gene expression in cultured cells [1][2][3][4][5][6] and transgenic mice. [7][8][9][10] In the absence of the antibiotic, expression of the target gene is induced.…”