Retroviral proteases (RPs) are of high interest owing to their crucial role in the maturation process of retroviral particles. RPs are obligatory homodimers, with a pepsin‐like active site built around two aspartates (in DTG triads) that activate a water molecule, as the nucleophile, under two flap loops. Mason–Pfizer monkey virus (M‐PMV) is unique among retroviruses as its protease is also stable in the monomeric form, as confirmed by an existing crystal structure of a 13 kDa variant of the protein (M‐PMV PR) and its previous biochemical characterization. In the present work, two mutants of M‐PMV PR, D26N and C7A/D26N/C106A, were crystallized in complex with a peptidomimetic inhibitor and one mutant (D26N) was crystallized without the inhibitor. The crystal structures were solved at resolutions of 1.6, 1.9 and 2.0 Å, respectively. At variance with the previous study, all of the new structures have the canonical dimeric form of retroviral proteases. The protomers within a dimer differ mainly in the flap‐loop region, with the most extreme case observed in the apo structure, in which one flap loop is well defined while the other flap loop is not defined by electron density. The presence of the inhibitor molecules in the complex structures was assessed using polder maps, but some details of their conformations remain ambiguous. In all of the presented structures the active site contains a water molecule buried deeply between the Asn26‐Thr27‐Gly28 triads of the protomers. Such a water molecule is completely unique not only in retropepsins but also in aspartic proteases in general. The C7A and C106A mutations do not influence the conformation of the protein. The Cys106 residue is properly placed at the homodimer interface area for a disulfide cross‐link, but the reducing conditions of the crystallization experiment prevented S—S bond formation. An animated Interactive 3D Complement (I3DC) is available in Proteopedia at http://proteopedia.org/w/Journal:Acta_Cryst_D:S2059798319011355.

中文翻译：

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单体和二聚体状态的逆转录病毒蛋白酶的比较。

逆转录病毒蛋白酶（RPs）由于其在逆转录病毒颗粒成熟过程中的关键作用而备受关注。RPs是必不可少的同二聚体，在两个天冬氨酸（在DTG三联体中）周围建立了胃蛋白酶样活性位点，该天冬氨酸在两个襟翼环下激活了作为亲核体的水分子。Mason-Pfizer猴病毒（M-PMV）在逆转录病毒中是独特的，因为其蛋白酶也以单体形式稳定，这已由该蛋白（M-PMV PR）的13 kDa变体的现有晶体结构及其先前的生化试剂证实表征。在目前的工作中，M-PMV PR的两个突变体D26N和C7A / D26N / C106A与拟肽抑制剂复合结晶，而一个突变体（D26N）在没有抑制剂的情况下结晶。分别以1.6，1.9和2.0Å的分辨率解析晶体结构。与以前的研究不同，所有新结构都具有逆转录病毒蛋白酶的规范二聚体形式。二聚体内的启动子主要在襟翼环区域不同，最极端的情况是在载脂蛋白结构中观察到，其中一个襟翼环定义良好，而另一个襟翼环则不受电子密度的限制。使用开拓地图评估了复杂结构中抑制剂分子的存在，但其构象的某些细节仍然不清楚。在所有提出的结构中，活性位点都包含一个深埋在前体Asn26-Thr27-Gly28三联体之间的水分子。这样的水分子不仅在逆转录胃蛋白酶中而且在通常的天冬氨酸蛋白酶中都是完全独特的。C7A和C106A突变不影响蛋白质的构象。将Cys106残基正确地放置在同二聚体交联区域的二硫键交联处，但是结晶实验的还原条件阻止了S键的形成。可以在Proteopedia中获得动画的交互式3D补语（I3DC），网址为http://proteopedia.org/w/Journal:Acta_Cryst_D:S2059798319011355。