Li Tongqing, Stayrook Steven E, Li Wenxue, Wang Yueyue, Li Hengyi, Zhang Jianan, Liu Yansheng, Klein Daryl E
Department of Pharmacology, Yale School of Medicine, New Haven, CT, USA.
Yale Cancer Biology Institute, Yale University, West Haven, CT, USA.
Nat Commun. 2025 Apr 15;16(1):3580. doi: 10.1038/s41467-025-58828-w.
Isthmin-1 (ISM1) is a recently described adipokine with insulin-like properties that can control hyperglycemia and liver steatosis. Additionally, ISM1 is proposed to play critical roles in patterning, angiogenesis, vascular permeability, and apoptosis. A key feature of ISM1 is its AMOP (adhesion-associated domain in MUC4 (Mucin-4) and other proteins) domain which is essential for many of its functions. However, the molecular details of AMOP domains remain elusive as there are no descriptions of their structure. Here we determined the crystal structure of ISM1 including its thrombospondin type I repeat (TSR) and AMOP domain. Interestingly, ISM1's AMOP domain exhibits a distinct fold with similarities to bacterial streptavidin. When comparing our structure to predicted structures of other AMOP domains, we observed that while the core streptavidin-like barrel is conserved, the surface helices and loops vary greatly. Thus, the AMOP domain fold allows for structural plasticity that may underpin its diverse functions. Furthermore, and contrary to prior studies, we show that highly purified ISM1 does not stimulate AKT phosphorylation on 3T3-F442A pre-adipocytes. Rather, we find that co-purifying growth factors are responsible for this activity. Together, our data reveal the structure and clarify functional studies of this enigmatic protein.
肌动蛋白-1（ISM1）是一种最近被描述的具有胰岛素样特性的脂肪因子，能够控制高血糖和肝脂肪变性。此外，ISM1被认为在模式形成、血管生成、血管通透性和细胞凋亡中发挥关键作用。ISM1的一个关键特征是其AMOP（MUC4（粘蛋白-4）和其他蛋白质中的粘附相关结构域）结构域，这对其许多功能至关重要。然而，由于没有关于AMOP结构域结构的描述，其分子细节仍然难以捉摸。在这里，我们确定了包括其血小板反应蛋白I型重复序列（TSR）和AMOP结构域的ISM1的晶体结构。有趣的是，ISM1的AMOP结构域呈现出与细菌抗生物素蛋白相似的独特折叠。当将我们的结构与其他AMOP结构域的预测结构进行比较时，我们观察到虽然核心的抗生物素蛋白样桶状结构是保守的，但表面螺旋和环却有很大差异。因此，AMOP结构域折叠允许结构可塑性，这可能是其多种功能的基础。此外，与先前的研究相反，我们表明高度纯化的ISM1不会刺激3T3-F442A前脂肪细胞上的AKT磷酸化。相反，我们发现共纯化的生长因子负责这种活性。总之，我们的数据揭示了这种神秘蛋白质的结构并阐明了其功能研究。
