An asparaginyl endopeptidase processes a microbial antigen for class II MHC presentation
An asparaginyl endopeptidase processes a microbial antigen for class II MHC presentation
复制标题
作者：
Manoury, B;Hewitt, EW;Watts, C
外来蛋白抗原必须在核内体或溶酶体内被分解以产生合适的肽，这些肽将与II类主要组织相容性复合体分子形成复合物以呈递给T细胞。然而，不知道抗原加工需要哪些蛋白酶。为了研究这一点，我们将微生物破伤风毒素抗原（TTCF）的一个结构域暴露于已从人B细胞系纯化的破坏的溶酶体中。在这里，我们表明，占主导地位的加工活动是不是一个已知的溶酶体组织蛋白酶，这通常被认为是参与抗原加工的主要酶，而是天冬酰胺特异性半胱氨酸内肽酶。这种酶似乎与最初在植物和寄生虫中发现的豆科蛋白/血红蛋白酶天冬酰胺酰内肽酶的哺乳动物同源物相似或相同（1）（3）。我们设计了B细胞天冬酰胺酰内肽酶（AEP）的竞争性肽抑制剂，其特异性地阻断其蛋白水解活性并抑制TTCF的体外加工。在体内，这些抑制剂减缓TTCF呈递给T细胞，而用AEP预处理TTCF加速其呈递，表明该酶在TTCF处理中执行关键步骤。我们还表明，N-糖基化的天冬酰胺残基块AEP行动在体外。这表明N-糖基化可以消除哺乳动物蛋白中AEP加工的位点，从而允许优先加工微生物抗原。
Foreign protein antigens must be broken down within endosomes or lysosomes to generate suitable peptides that will form complexes with class II major histocompatibility complex molecules for presentation to T cells. However, it is not known which proteases are required for antigen processing. To investigate this, we exposed a domain of the microbial tetanus toxin antigen (TTCF) to disrupted lysosomes that had been purified from a human B-cell line. Here we show that the dominant processing activity is not one of the known lysosomal cathepsins, which are generally believed to be the principal enzymes involved in antigen processing, but is instead an asparagine-specific cysteine endopeptidase. This enzyme seems similar or identical to a mammalian homologue(1) of the legumain/haemoglobinase asparaginyl endopeptidases found originally in plants' and parasites(3). We designed competitive peptide inhibitors of B-cell asparaginyl endopeptidase (AEP) that specifically block its proteolytic activity and inhibit processing of TTCF in vitro. In vivo, these inhibitors slow TTCF presentation to T cells, whereas preprocessing of TTCF with AEP accelerates its presentation, indicating that this enzyme performs a key step in TTCF processing. We also show that N-glycosylation of asparagine residues blocks AEP action in vitro. This indicates that N-glycosylation could eliminate sites of processing by AEP in mammalian proteins, allowing preferential processing of microbial antigens.