TERTIARY STRUCTURES OF ACTIVIN AND ACTIVIN RECEPTORS

本研究的目的是了解生物信号如何在多种靶细胞的膜上转导。神经内分泌肽和激活素发挥着广泛的生物学功能，为研究这些机制提供了一个很好的系统。两种类型的受体参与触发信号级联在他们的顺序结合激活配体。这些是跨膜蛋白，每个都有一个跨膜螺旋和一个细胞质丝氨酸/苏氨酸激酶结构域。我们的目标是从结构上理解激活素与第一受体（II型激活素受体，ActRII）的胞外结构域的结合如何被转导到信号通路的下游组分，以激活I型受体（ActRII）的激酶活性，该受体与激活素/ActRII复合物复合物。我们的主要重点是确定细胞外，配体结合域的ActRII （ActRII- ecd）与激活素配体，或ActRII- ecd，或两者的复合物的结构。我们目前已经获得了与激活素配体配合的ActRII-ECD单晶。我们还建立了ActRII-ECD的高水平表达以获得单晶。基于不同配体/受体复合物的化学计量比和结合亲和力，其中异二聚体（不对称）或同二聚体（对称）配体，如抑制素或激活素，分别与两种不同类型受体的寡聚物（不对称）相互作用，我们将建立该过程的热力学和动力学速率。在我们之前对Act iii - ecd的1.5 A晶体结构的研究基础上，我们希望提供一个结构基础，将配体和受体组分之间的复合物形成引起的构象变化与跨膜信号传导机制联系起来。重组形式的激活素，抑制素（激活素拮抗剂），ActRII-ECD现在有足够的数量来进行各种配合物的化学表征和晶体结构测定。研究结果将为确定这种普遍存在的神经肽的结合特异性提供坚实的基础，从而有助于设计有效和持久的拟肽药物，用于调节各种激素相关疾病和控制人口。

The goal of the proposed study is to understand how biological signal is transduced across the membrane of a variety of target cells. Neuroendocrine peptides, activins exert a wide range of biological functions, which provides an excellent system to study such mechanisms. Two types of receptors are involved to trigger the signaling cascade upon their sequential binding to activin ligand. These are transmembrane proteins each with a single transmembrane helix and a cytoplasmic Ser/Thr kinase domain. Our goal is to have a structural understanding of how the binding of activin to the extracellular domain of the first receptor (type II activin receptor, ActRII) is then transduced to the downstream components of the signaling pathway to activate the kinase activity of the type I receptor (ActRII), which complexes with activin/ActRII complex. Our primary focus is on the structure determination of the extracellular, ligand-binding domain of ActRII (ActRII-ECD) in complex with either activin ligand, or ActRII-ECD, or both. We have currently obtained single crystals of ActRII-ECD in complex with activin ligand. We have also established high-level expression of ActRII-ECD to obtain single crystals. Based on stoichiometric ratios and binding affinity of different ligand/receptor complexes, in which either a heterodimeric (asymmetric) or homodimeric (symmetric) ligands, such as inhibin or activin, respectively, interacts with oligomeric complexes of two different types of receptor (asymmetric), we will establish the thermodynamic and kinetic rates of the processes. Expanding on our previous work on 1.5 A crystal structure of Act RII-ECD alone, we hope to provide a structural basis to correlate conformational changes induced upon complex formation between the ligand and receptor components to the transmembrane signaling mechanism. Recombinant forms of activins, inhibins (activin antagonist), ActRII-ECD are now available in large enough quantities to carry out chemical characterization and crystal structure determination of various complexes. Results will provide a firm basis to characterize the binding specificity of this ubiquitous neuropeptide, from which we can facilitate the design of effective and long-lasting peptidomimetic drugs to be used in modulating various hormone-related diseases and in population control.