Peptide trimming by endoplasmic reticulum aminopeptidases: Role of MHC class I binding and ERAP dimerization

Presentation of short peptides, produced through intracellular proteolysis, by MHC class I molecules (MHC-I) is the basis of adaptive immune surveillance and responses by cytolytic CD8⁺ T lymphocytes. In the principal pathway of peptide processing for MHC-I that operates in all nucleated cells, MHC-I-binding peptides are produced through stepwise proteolysis starting with source protein degradation by cytosolic proteasome complexes. Among the fraction of proteasome products reaching the lumen of the endoplasmic reticulum, a significant proportion is thought to have a length exceeding that adapted to MHC class I binding and requires N-terminal trimming. This is carried out by one murine and two human endoplasmic reticulum aminopeptidases, the ERAP enzymes. While the critical role of ERAP for producing a ligandome optimized for MHC-I is well documented, it remains unclear how this is mechanistically achieved. In this review, we will discuss the evidence supporting the alternative "MHC template" and "molecular ruler" models that have been proposed to explain how ERAP activity adapts to the ligand requirements of MHC-I. We will also review evidence for dimerization of the two human ERAP enzymes and its potential functional relevance.