Major histocompatibility complex class I-related protein 1 molecule (MR1) presents bacteria-derived vitamin B metabolites that are synthesised by a range of microbes. Presentation of MR1-metabolite complexes by the infected cells activates the highly abundant innate-like T cells, mucosal associated invariant T (MAIT) cells. Activated MAIT cells secrete inflammatory cytokines and acquire cytotoxic activity to clear the infection.
MR1 is maintained intracellularly as an endoplasmic reticulum (ER)-resident pool in the absence of infection, but encounter of metabolic ligands induces the trafficking of MR1 to the cell surface. MR1-ligand complexes stay on the cell surface for several hours after which they are internalised and mostly degraded. Elimination of surface MR1 is a requirement to terminate MAIT cell responses, so the cellular machinery that controls the internalisation of MR1 plays a critical role in the regulation of the MR1-MAIT cell axis.
We have employed a genome-wide CRISPR-Cas9 library screen to identify proteins that control MR1 internalisation, and we identified adaptor protein complex 2 (AP2) as a potential regulator of this process. To verify the significance of AP2 in MR1 internalisation, we knocked out AP2 gene using CRISPR-Cas9 gene editing. In the AP2 knock out cells, we observed higher MR1 surface expression level with lower MR1 internalisation rate. Interestingly though, the peptide sequence of the MR1 cytoplasmic tail does not contain the canonical sorting motif common to other AP2 substrates. In conclusion, MR1 internalisation is regulated by AP2 using a novel recognition motif. These results open the possibility of manipulating the internalisation of MR1 without affecting the trafficking properties of most AP2-regulated membrane proteins