Titolo della tesi: Molecular characterization of a “short” form of the amino peptidase ERAP2 that binds IRAP: functional implications
The aminopeptidases ERAP1, ERAP2, and IRAP play a crucial role in the antigen presentation by HLA class I molecules and thus in the activation of the specific immune response mediated by CD8+ T lymphocytes by refining the peptidome in either the ER (ERAP1 and ERAP2) or the endosomes (IRAP). The three gene are closely related and lie in the same region in chromosome 5. The analysis on their expression across the zoological scale suggests that IRAP has been the first gene to appear and that the other two aminopeptidases could possibly be derived from gene duplication events. In humans however, ERAP1 and IRAP are commonly expressed while ERAP2 is under balanced selection that maintains two different haplotypes, one of which encodes for the canonical protein, whereas the other encodes for a mRNA undergoing to No-Sense Mediated Decay (NMD). ERAP2 is absent in rodents as well as, due to NMD, in about 25% of the human population. Therefore, this gene appears to be non-essential, nevertheless it is maintained in humans. In this thesis, I show that macrophages, but not monocytes or other blood mononuclear cells, express and secrete an N-terminus “short” form of ERAP2 which is expressed independently from gene variations. The generation of this "short" ERAP2 occurs in an acidic microenvironment that results in the opening of a structural motif peculiar to ERAP2 via an autocatalytic cleavage mechanism. In addition, this "short" ERAP2 directly binds IRAP also known as Ang IV receptor (AT4R) and the two molecules are co-expressed both in the endosomes as well as in the cell membrane. These data prompt to reconsider the role of ERAP2 which, besides being involved in antigen presentation, as “short” form could interfere with the renin-angiotensin system by binding IRAP. I finally speculate that this could be the ultimate reason why ERAP2 has been conserved in humans.