Institute of Experimental Hematology and Transfusion medicine, University Hospital of Bonn, Sigmund-Freud Street 25, 53127 Bonn, Germany.
CovalX, Schützengasse 2, CH-8001 Zürich, Switzerland.
Biomolecules. 2019 Nov 21;9(12):765. doi: 10.3390/biom9120765.
Factor XIII (FXIII) is a predominant determinant of clot stability, strength, and composition. Plasma FXIII circulates as a pro-transglutaminase with two catalytic A subunits and two carrier-protective B subunits in a heterotetramer (FXIII-AB). FXIII-A and -B subunits are synthesized separately and then assembled in plasma. Following proteolytic activation by thrombin and calcium-mediated dissociation of the B subunits, activated FXIII (FXIIIa) covalently cross links fibrin, promoting clot stability. The zymogen and active states of the FXIII-A subunits have been structurally characterized; however, the structure of FXIII-B subunits and the FXIII-AB complex have remained elusive. Using integrative hybrid approaches including atomic force microscopy, cross-linking mass spectrometry, and computational approaches, we have constructed the first all-atom model of the FXIII-AB complex. We also used molecular dynamics simulations in combination with isothermal titration calorimetry to characterize FXIII-AB assembly, activation, and dissociation. Our data reveal unequal pairing of individual subunit monomers in an otherwise symmetric complex, and suggest this unusual structure is critical for both assembly and activation of this complex. Our findings enhance understanding of mechanisms associating FXIII-AB mutations with disease and have important implications for the rational design of molecules to alter FXIII assembly or activity to reduce bleeding and thrombotic complications.
凝血因子 XIII（FXIII）是决定血栓稳定性、强度和组成的主要因素。血浆 FXIII 以具有两个催化 A 亚基和两个载体保护 B 亚基的前转谷氨酰胺酶形式循环，形成四聚体（FXIII-AB）。FXIII-A 和 -B 亚基分别合成，然后在血浆中组装。在凝血酶的蛋白水解激活和钙介导的 B 亚基解离后，激活的 FXIII（FXIIIa）共价交联纤维蛋白，促进血栓稳定性。FXIII-A 亚基的酶原和活性状态已得到结构表征；然而，FXIII-B 亚基和 FXIII-AB 复合物的结构仍然难以捉摸。我们使用整合的混合方法，包括原子力显微镜、交联质谱和计算方法，构建了 FXIII-AB 复合物的第一个全原子模型。我们还使用分子动力学模拟结合等温滴定量热法来表征 FXIII-AB 的组装、激活和解离。我们的数据揭示了在对称复合物中单个亚基单体的不等配对，并且表明这种不寻常的结构对于该复合物的组装和激活都是至关重要的。我们的研究结果增强了对与 FXIII-AB 突变相关疾病的机制的理解，并对改变 FXIII 组装或活性以减少出血和血栓并发症的分子的合理设计具有重要意义。
