Nissen Mark S, Kumar G N Mohan, Youn Buhyun, Knowles D Benjamin, Lam Ka Sum, Ballinger W Jordan, Knowles N Richard, Kang Chulhee
School of Molecular Biosciences, Washington State University, Pullman, Washington 99164-4660, USA.
Plant Cell. 2009 Mar;21(3):861-75. doi: 10.1105/tpc.108.064717. Epub 2009 Mar 20.
Potato (Solanum tuberosum) multicystatin (PMC) is a crystalline Cys protease inhibitor present in the subphellogen layer of potato tubers. It consists of eight tandem domains of similar size and sequence. Our in vitro results showed that the pH/PO(4)(-)-dependent oligomeric behavior of PMC was due to its multidomain nature and was not a characteristic of the individual domains. Using a single domain of PMC, which still maintains inhibitor activity, we identified a target protein of PMC, a putative Cys protease. In addition, our crystal structure of a representative repeating unit of PMC, PMC-2, showed structural similarity to both type I and type II cystatins. The N-terminal trunk, alpha-helix, and L2 region of PMC-2 were most similar to those of type I cystatins, while the conformation of L1 more closely resembled that of type II cystatins. The structure of PMC-2 was most similar to the intensely sweet protein monellin from Dioscorephyllum cumminisii (serendipity berry), despite a low level of sequence similarity. We present a model for the possible molecular organization of the eight inhibitory domains in crystalline PMC. The unique molecular properties of the oligomeric PMC crystal are discussed in relation to its potential function in regulating the activity of proteases in potato tubers.
马铃薯（Solanum tuberosum）多胱抑素（PMC）是一种存在于马铃薯块茎亚木栓形成层的结晶性半胱氨酸蛋白酶抑制剂。它由八个大小和序列相似的串联结构域组成。我们的体外实验结果表明，PMC的pH/PO(4)(-)依赖性寡聚行为归因于其多结构域性质，而非单个结构域的特性。利用仍保持抑制活性的PMC单个结构域，我们鉴定出了PMC的一个靶蛋白，一种假定的半胱氨酸蛋白酶。此外，我们测定的PMC代表性重复单元PMC-2的晶体结构显示，它与I型和II型胱抑素均存在结构相似性。PMC-2的N端主干、α螺旋和L2区域与I型胱抑素最为相似，而L1的构象则更类似于II型胱抑素。尽管序列相似性较低，但PMC-2的结构与来自Dioscorephyllum cumminisii（甜叶悬钩子，即甜叶莓）的甜味蛋白莫内林最为相似。我们提出了一个关于结晶PMC中八个抑制结构域可能的分子组织模型。结合其在调节马铃薯块茎中蛋白酶活性方面的潜在功能，讨论了寡聚PMC晶体独特的分子特性。
