Crystal structure of scabies mite inactivated protease paralogue S-D1 (SMIPP-S-D1)

The scabies mite (Sarcoptes scabiei) is a parasitic mite responsible for major morbidity in disadvantaged communities and immuno-compromised patients worldwide. In addition to the physical discomfort caused by the disease, scabies infestations facilitate infection by Streptococcal species via skin lesions, resulting in a high prevalence of rheumatic fever/heart disease in affected communities. The scabies mite produces 33 proteins that are closely related to the dust mite group 3 allergen and belong to the S1-like protease family (chymotrypsin-like). However, all but one of these molecules contain mutations in the conserved active-site catalytic triad that are predicted to render them catalytically inactive. These molecules are thus termed Scabies Mite Inactivated Protease Paralogues (SMIPPs). The precise function of SMIPPs remains unclear. However, it has been suggested that these proteins may function by binding and protecting target substrates from cleavage by host immune proteases, thus preventing the host from mounting an effective immune challenge. In order to begin to understand the structural basis for SMIPP function, we solved the crystal structures of SMIPP-S-I1 and SMIPP-S-D1 at 1.85 and 2.0 A resolution respectively. Both structures adopt the characteristic serine protease fold, albeit with large structural variations over much of the molecule. In both structures, mutations in the catalytic triad together with occlusion of the S1 subsite by a conserved Tyr200 residue is predicted to block substrate ingress. Accordingly, we show that both proteases lack catalytic function. Attempts to restore function (via site directed mutagenesis of catalytic residues as well as Tyr200) were unsuccessful. Taken together, these data suggest that SMIPPs have lost the ability to bind substrates in a classical "canonical" fashion, and instead have evolved alternative functions in the lifecycle of the Scabies mite. To cite this data use the following DOI: 10.4225/52/557FA7A6C8287

疥螨（Sarcoptes scabiei）是一种寄生性螨类，可在全球范围内的弱势社群与免疫受损患者群体中造成严重的发病负担。除该疾病引发的躯体不适外，疥螨感染还会通过皮肤破溃部位继发链球菌属物种（Streptococcal species）感染，致使受影响社群中风湿热/心脏病的患病率显著升高。疥螨可产生33种与尘螨第3组变应原（dust mite group 3 allergen）高度同源的蛋白，此类蛋白隶属于S1样蛋白酶家族（S1-like protease family，类胰凝乳蛋白酶型）。然而除其中1种外，其余所有这类分子均在保守的活性位点催化三联体区域存在突变，经预测会使其丧失催化活性。这类分子因此被命名为疥螨灭活蛋白酶旁系同源物（Scabies Mite Inactivated Protease Paralogues，简称SMIPPs）。SMIPPs的确切功能目前仍不明确。不过有研究推测，这类蛋白可通过结合并保护靶底物免受宿主免疫蛋白酶的切割，从而阻断宿主发起有效的免疫攻击。为了阐明SMIPPs功能的结构基础，我们分别以1.85 Å和2.0 Å的分辨率解析了SMIPP-S-I1与SMIPP-S-D1的晶体结构。两种结构均呈现出典型的丝氨酸蛋白酶折叠构象，但在分子的大部分区域均存在显著的结构变异。在两种结构中，催化三联体的突变，加上保守的Tyr200残基对S1亚位点的遮蔽，经预测会阻断底物的进入通道。据此我们证实，这两种蛋白酶均不具备催化活性。后续我们尝试通过对催化残基及Tyr200进行定点诱变以恢复其功能，但均未获得成功。综合以上实验结果，这些数据表明SMIPPs已丧失了以经典‘规范’方式结合底物的能力，转而在疥螨的生命周期中演化出了其他功能。引用此数据集请使用以下DOI：10.4225/52/557FA7A6C8287