The recently discovered neutrophil serine protease 4 (NSP4) is the fourth member of the NSP family, which includes the well-studied neutrophil elastase, proteinase 3 and cathepsin G. Like the other three NSP members, NSP4 is synthesized by myeloid precursors in the bone marrow and, after cleavage of the two-amino acid activation peptide, is stored as an active protease in azurophil granules of neutrophils. Based on its primary amino acid sequence, NSP4 is predicted to have a shallow S1 specificity pocket with elastase-like substrate specificity. However, NSP4 was found to preferentially cleave after an arginine residue. Structural studies resolved this paradox by revealing an unprecedented mechanism of P1-arginine recognition. In contrast to the canonical mechanism in which the P1-arginine residue points down into a deep S1 pocket, the arginine side chain adopts a surface-exposed 'up' conformation in the NSP4 active site. This conformation is stabilized by the Phe190 residue, which serves as a hydrophobic platform for the aliphatic portion of the arginine side chain, and a network of hydrogen bonds between the arginine guanidium group and the NSP4 residues Ser192 and Ser216. This unique configuration allows NSP4 to cleave even after naturally modified arginine residues, such as citrulline and methylarginine. This non-canonical mechanism, characterized by the hallmark 'triad' Phe190-Ser192-Ser216, is largely preserved throughout evolution starting with bony fish, which appeared about 400 million years ago. Although the substrates and physiological role of NSP4 remain to be determined, its remarkable evolutionary conservation, restricted tissue expression and homology to other neutrophil serine proteases anticipate a function in immune-related processes.

中文翻译：

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精氨酸特异性底物裂解的古老机制：NSP4的“作用”是什么？

最近发现的嗜中性粒细胞丝氨酸蛋白酶4（NSP4）是NSP家族的第四个成员，其中包括经过研究的嗜中性粒细胞弹性蛋白酶，蛋白酶3和组织蛋白酶G。与其他三个NSP成员一样，NSP4由骨骼中的髓样前体合成。并在切割两个氨基酸的活化肽后，将其作为活性蛋白酶存储在嗜中性粒细胞的嗜酸性粒状颗粒中。基于其一级氨基酸序列，NSP4预计具有较浅的S1特异性口袋，并具有类似弹性蛋白酶的底物特异性。然而，发现NSP4在精氨酸残基之后优先裂解。结构研究通过揭示P1-精氨酸识别的前所未有的机制解决了这一悖论。与P1精氨酸残基指向深S1口袋的规范机制相反，精氨酸侧链在NSP4活性位点采用表面暴露的“向上”构象。该构象通过Phe190残基（用作精氨酸侧链的脂族部分的疏水平台）以及精氨酸胍基团与NSP4残基Ser192和Ser216之间的氢键网络稳定。这种独特的配置使NSP4甚至在天然修饰的精氨酸残基（例如瓜氨酸和甲基精氨酸）裂解后也能裂解。这种非经典机制的特征是“三合会” Phe190-Ser192-Ser216，在始于约4亿年前的骨鱼开始的整个进化过程中都得到了很大的保留。尽管NSP4的底物和生理作用尚待确定，但其显着的进化保守性，