ER aminopeptidase 1 (ERAP1) customizes antigenic peptide precursors for MHC class I presentation and edits the antigenic peptide repertoire. Coding single nucleotide polymorphisms (SNPs) in ERAP1 were recently linked with predisposition to autoimmune disease, suggesting a link between pathogenesis of autoimmunity and ERAP1-mediated Ag processing. To investigate this possibility, we analyzed the effect that disease-linked SNPs have on Ag processing by ERAP1 in vitro. Michaelis–Menten analysis revealed that the presence of SNPs affects the Michaelis constant and turnover number of the enzyme. Strikingly, specific ERAP1 allele-substrate combinations deviate from standard Michaelis–Menten behavior, demonstrating substrate-inhibition kinetics; to our knowledge, this phenomenon has not been described for this enzyme. Cell-based Ag-presentation analysis was consistent with changes in the substrate inhibition constant Ki, further supporting that ERAP1 allelic composition may affect Ag processing in vivo. We propose that these phenomena should be taken into account when evaluating the possible link between Ag processing and autoimmunity.

The adipocyte-derived leucine aminopeptidase (A-LAP)/ER aminopeptidase-1 is a multi-functional enzyme belonging to the M1 family of aminopeptidases. It was reported that the polymorphism Lys528Arg in the human A-LAP gene is associated with essential hypertension. In this study, the role of Lys528 in the enzymatic activity of human A-LAP was examined by site-directed mutagenesis. Among non-synonymous polymorphisms tested, only Lys528Arg reduced enzymatic activity. The replacement of Lys528 with various amino acids including Ala, Met, His and Arg caused a significant decrease in the enzymatic activity. Molecular modeling of the enzyme suggested that Lys528 is located near the entrance of the substrate pocket. These results suggest that Lys528 is important for maximal activity of A-LAP by maintaining the appropriate structure of the substrate pocket of the enzyme. The reduced enzymatic activity of A-LAP may cause high blood pressure and the observed association between the polymorphism and hypertension.

An interferon-induced endoribonuclease, ribonuclease L (RNase L), is implicated in both the molecular mechanism of action of interferon and the fundamental control of RNA stability in mammalian cells. RNase L is catalytically active only after binding to an unusual activator molecule containing a 5 0 -phosphorylated 2 0 ,5 0 -linked oligoadenylate (2-5A), in the N-terminal half. Here, we report the crystal structure of the N-terminal ankyrin repeat domain (ANK) of human RNase L complexed with the activator 2-5A. This is the first structural view of an ankyrin repeat structure directly interacting with a nucleic acid, rather than with a protein. The ANK domain folds into eight ankyrin repeat elements and forms an extended curved structure with a concave surface. The 2-5A molecule is accommodated at a concave site and directly interacts with ankyrin repeats 2-4. Interestingly, two structurally equivalent 2-5A binding motifs are found at repeats 2 and 4. The structural basis for 2-5A recognition by ANK is essential for designing stable 2-5As with a high likelihood of activating RNase L.

Endoplasmic reticulum aminopeptidase 1 (ERAP1) is a multifunctional enzyme with an important role in processing antigenic peptides presented to class I major histocompatibility complex in the endoplasmic reticulum. In this study, we found that endoplasmic reticulum-retained ERAP1 was secreted from macrophages in response to activation by treatment with lipopolysaccharide (LPS) and interferon (IFN)-␥ and enhanced their phagocytic activity. Enhancement of the phagocytic activity of murine macrophage RAW264.7 cells induced by LPS/ IFN-␥ was inhibited by a potent aminopeptidase inhibitor, amastatin. The addition of recombinant wild-type but not inactive mutant ERAP1 to culture medium enhanced phagocytosis. These results suggest that enhancement of phagocytic activity is at least in part mediated by secreted ERAP1 through the generation of active peptides processed by the enzyme. Our data reveal ERAP1-mediated activation of macrophages for the first time and will provide new insights into the role of this enzyme in innate immunity.It is well known that endoplasmic reticulum aminopeptidase 1 (ERAP1) is a multifunctional enzyme belonging to the M1 family of aminopeptidases with roles in the regulation of blood pressure, angiogenesis, ectodomain shedding of several cytokine receptors, and processing of antigenic peptides presented to MHC class I molecules (1-4). Its cDNA was initially cloned as adipocyte-derived leucine aminopeptidase (5). Based on its multifunctional properties, adipocyte-derived leucine aminopeptidase is also designated ERAP1, ERAAP (endoplasmic reticulum aminopeptidase associated with antigen presentation), PILSAP (puromycin-insensitive leucine-specific aminopeptidase), and ARTS-1 (aminopeptidase regulator of TNFR1 shedding) (6 -9) (in this paper, ERAP1 is used hereafter). Although it is evident that ERAP1 plays important roles in several pathophysiological processes, its subcellular localization is still under debate. Although several reports have presented evidence showing its localization in the ER (6, 7) or cytoplasm (10) as a soluble protein, others have shown it on the cell surface as a type II membrane-spanning protein (9).ERAP1 is a monomeric zinc-metallopeptidase that shows a preference for leucine when measured by synthetic substrates (5). On the other hand, it shows relatively broad substrate specificity toward natural peptide hormones, such as angiotensin II, kallidin, and neurokinin A, which may reflect its role in the processing of various precursors of antigenic peptides presented to MHC class I molecules (11). On the basis of a preference for substrates of a specific length and C-terminal hydrophobic amino acid, the "molecular ruler" mechanism was proposed for the processing of antigenic peptides by the enzyme (12).Because ERAP1 inactivates angiotensin II and converts kallidin to bradykinin, it was initially speculated that it might regulate blood pressure (11). Subsequently, by screening for polymorphisms in the human ERAP1 gene, Yamamoto et al. (13) identified an association of K5...