The Cytoplasmic Domain of MHC Class I Molecules as a Molecular Switch: A Perspective from Short Linear Motifs and...

Classical Major Histocompatibility Complex Class I (MHC-I) molecules are traditionally viewed as stable peptide-presenting structures expressed on the surface of all nucleated cells. Their expression by professional antigen-presenting dendritic cells (DCs) enables CD8+ T-cell activation, differentiation, and immune surveillance. However, accumulating evidence indicates that cell-surface MHC-I molecules exist in three major conformational states: (1) β2m-associated, peptide-loaded conformers that originate in the endoplasmic reticulum and pass through the Golgi apparatus after binding proteasome-generated cytosolic peptides (hereafter referred to as closed conformers); (2) β2m-free, peptide-empty conformers that arise following β2m dissociation from closed conformers either at the plasma membrane or after internalization and recycling (hereafter referred to as open conformers); and (3) β2m-associated, peptide-empty conformers that represent an intermediate state between closed and open conformers. Here, we propose a conceptual framework, supported by computational predictors of intrinsically disordered regions, in which transitions between closed and open MHC-I conformers are coupled to intracellular regulatory processes, including post-translational modifications of conserved motifs, intracellular trafficking, and signaling. Although direct experimental evidence linking these processes remains limited, we integrate independent observations into a working model that may guide future investigations into MHC-I-mediated cell-cell communication in both immune and non-immune contexts, in health and disease. For clarity, in this article we define "open conformers" as structurally competent, β2m-free, and peptide-deficient MHC-I molecules.