Structure-fonction de MARCH1, une E3 ubiquitine ligase régulant la présentation antigénique par le CMH II

Classical MHC class II molecules are responsible for the presentation of exogenous peptides to CD4+ T cells, which is essential for the establishment of the adaptive immune response. However, problems with recognition of auto-antigens and the subsequent cell elimination are at the root of numerous autoimmune diseases. Manipulation of the antigen presentation pathway in order to eliminate cells that present self-antigens could serve as potential treatments of many autoimmune disorders. It is therefore essential to deepen our knowledge regarding the mechanisms regulating antigen presentation.  Antigen presentation is regulated both transcriptionally and post-translationally. Whereas many cytokines affect the latter, IL-10, an anti-inflammatory cytokine, causes the intracellular retention of MHC II molecules. This phenotype is the result of the ubiquitination of MHC II -chain cytoplasmic tail by MARCH1.  MARCH1 is an E3 ubiquitin ligase expressed in secondary lymphoid organs. Until recently, little was known about the structure-function and the targets of MARCH1. Considering its major impact on antigen presentation, we were interested to study this E3 ligase in order to reveal how it is regulated and what are the required conditions for its function. In a first report, we have investigated the regulation of MARCH1’s protein expression. Our results revealed its autoregulation via dimer formation and autoubiquitination. In addition, we have demonstrated the involvement of MARCH1’s transmembrane domains for dimerization and MHC II interaction. In a second article, we highlighted the importance of MARCH1 localization for its function. Our results indicated that localization motifs in the C-terminal portion of MARCH1 were functional and revealed the presence of some sorting elements in the N-terminal portion of the molecule. A panel of mutant were used and allowed us to identify a ‘’VQNC’’ motif, located in the C-terminal cytoplasmic portion of MARCH1, in which the valine is central for the molecule’s function. Indeed, point-mutation of the valine led to a decrease in MARCH1 ability to relocate MHC II whereas BRET experiments revealed a modification in the spatial organization of the cytoplasmic tails. Moreover, a blast of sequence homology showed the presence or that same motif in others ubiquitine ligases, one of which is Parkin. Parkin is highly expressed in the brain and seems to be implicated in protein aggregates’ degradation. It was reported that malfunction of Parkin leads to the accumulation of aggregates, called Lewy bodies, responsible for the neural functions deficiencies observed in patients with Parkinson disease. Interestingly, a family for which the sickness was genetically transmitted has a mutated valine in the VQNC motif. We believe that the importance of this motif is not restricted to MARCH1 and could be generalized to others E3 ubiquitin ligases. This project enabled us to characterize the regulation mechanisms of MARCH1. In addition, we discovered various structural elements required for its function. Altogether, our data allows for a better understanding of the mechanisms controlling MHC II molecules antigen presentation.

Keyphrases: MARCH1, ubiquitination, autoregulation, dimerization, localization