MITOL regulates endoplasmic reticulum-mitochondria contacts via Mitofusin2.

The mitochondrial ubiquitin ligase MITOL regulates mitochondrial dynamics. We report here that MITOL regulates mitochondria-associated endoplasmic reticulum (ER) membrane (MAM) domain formation through mitofusin2 (Mfn2). MITOL interacts with and ubiquitinates mitochondrial Mfn2, but not ER-associated Mfn2.

Ubiquitination of CD86 is a key mechanism in regulating antigen presentation by dendritic cells.

Dendritic cells (DCs) require costimulatory molecules such as CD86 to efficiently activate T cells for the induction of adaptive immunity. DCs maintain minimal levels of CD86 expression at rest, but upregulate levels upon LPS stimulation. LPS-stimulated DCs produce the immune suppressive cytokine IL-10 that acts in an autocrine manner to regulate CD86 levels.

Ubiquitination regulates MHC class II-peptide complex retention and degradation in dendritic cells.

The expression and turnover of MHC class II-peptide complexes (pMHC-II) on the surface of dendritic cells (DCs) is essential for their ability to activate CD4 T cells efficiently. The half-life of surface pMHC-II is significantly greater in activated (mature) DCs than in resting (immature) DCs, but the molecular mechanism leading to this difference remains unknown. We now show that ubiquitination of pMHC-II by the E3 ubiquitin ligase membrane-associated RING-CH 1 (March-I) regulates surface expression, intracellular distribution, and survival of pMHC-II in DCs.

Contribution of lysine 11-linked ubiquitination to MIR2-mediated major histocompatibility complex class I internalization.

The polyubiquitin chain is generated by the sequential addition of ubiquitin moieties to target molecules, a reaction between specific lysine residues that is catalyzed by E3 ubiquitin ligase. The Lys(48)-linked and Lys(63)-linked polyubiquitin chains are well established inducers of proteasome-dependent degradation and signal transduction, respectively. The concept has recently emerged that polyubiquitin chain-mediated regulation is even more complex because various types of atypical polyubiquitin chains have been discovered in vivo.

MARCH-I: a new regulator of dendritic cell function.

We and other groups have demonstrated that the expression level of MHC class II (MHC II) is regulated through ubiquitination of the MHC II beta chain. We also reported that MARCH-I, an E3 ubiquitin ligase, is critical for this process. At present, however, the importance of MARCH-I-mediated MHC II regulation in vivo is still unknown.

Cutting edge: requirement of MARCH-I-mediated MHC II ubiquitination for the maintenance of conventional dendritic cells.

MARCH-I (membrane-associated RING-CH I) has been suggested as a physiological E3 ubiquitin ligase for both MHC class II (MHC II) and B7-2. In this study, we show that MARCH-I-mediated MHC II ubiquitination is necessary for the maintenance of conventional dendritic cell (cDC) functions in the steady state. MARCH-I-deficient cDCs accumulated MHC II and B7-2 and exhibited low Ag-presenting ability for exogenous Ags and low cytokine-producing ability upon stimulation in vivo.

E3 ubiquitin ligases for MHC molecules.

Recently, novel E3 ubiquitin ligases that target MHC molecules for lysosomal degradation have been discovered by several groups. All these E3s are membrane-bound and possess a variant type RING domain, termed the RING-CH or RING variant (RINGv) domain. They belong to a new E3 family designated Modulator of Immune Recognition (MIR), based on the name of the first identified family members.

Differential MHC class II synthesis and ubiquitination confers distinct antigen-presenting properties on conventional and plasmacytoid dendritic cells.

The importance of conventional dendritic cells (cDCs) in the processing and presentation of antigen is well established, but the contribution of plasmacytoid dendritic cells (pDCs) to these processes, and hence to T cell immunity, remains unclear. Here we showed that unlike cDCs, pDCs continued to synthesize major histocompatibility complex (MHC) class II molecules and the MHC class II ubiquitin ligase MARCH1 long after activation. Sustained MHC class II-peptide complex formation, ubiquitination and turnover rendered pDCs inefficient in the presentation of exogenous antigens but enabled pDCs to continuously present endogenous viral antigens in their activated state.

An excellent monitoring system for surface ubiquitination-induced internalization in mammals.

Background: At present, it is difficult to visualize the internalization of surface receptors induced by ubiquitination that is taken place at the plasma membrane in mammals. This problem makes it difficult to reveal molecular basis for ubiquitination-mediated internalization in mammals.

Methodology/principle Findings: In order to overcome it, we have generated T-REx-c-MIR, a novel mammalian Tet-on B cell line using a constitutively active E3 ubiquitin ligase, c-MIR, and its artificial target molecule.

Novel regulation of MHC class II function in B cells.

The presence of post-translational regulation of MHC class II (MHC II) under physiological conditions has been demonstrated recently in dendritic cells (DCs) that potently function as antigen-presenting cells (APCs). Here, we report that MARCH-I, an E3 ubiquitin ligase, plays a pivotal role in the post-translational regulation of MHC II in B cells. MARCH-I expression was particularly high in B cells, and the forced expression of MARCH-I induced the ubiquitination of MHC II.

A novel family of membrane-bound E3 ubiquitin ligases.

A novel E3 ubiquitin ligase family that consists of viral E3 ubiquitin ligases (E3s) and their mammalian homologues was recently discovered. These novel E3s are membrane-bound molecules that share the secondary structure and catalytic domain for E3 activity. All family members have two transmembrane regions at the center and a RING-CH domain at the amino terminus.

A novel mitochondrial ubiquitin ligase plays a critical role in mitochondrial dynamics.

In this study, we have identified a novel mitochondrial ubiquitin ligase, designated MITOL, which is localized in the mitochondrial outer membrane. MITOL possesses a Plant Homeo-Domain (PHD) motif responsible for E3 ubiquitin ligase activity and predicted four-transmembrane domains. MITOL displayed a rapid degradation by autoubiquitination activity in a PHD-dependent manner.

Inhibition of MHC class II expression and immune responses by c-MIR.

We previously reported a novel E3 ubiquitin ligase (E3), designated as c-MIR, which targets B7-2 to lysosomal degradation and down-regulates the B7-2 surface expression through ubiquitination of its cytoplasmic tail. B7-2 is well known as a costimulatory molecule for Ag presentation, suggesting that the manipulation of c-MIR expression modulates immune responses in vivo. To examine this hypothesis, we generated genetically modified mice in which c-MIR was expressed under an invariant chain (Ii) promoter.