Structural insights into the unique activation mechanisms of receptor tyrosine kinases

受体酪氨酸激酶 (RTK) 在调节正常细胞过程中发挥关键作用，并与许多细胞相关 人类疾病。每个 RTK 原聚体都包含一个结合激活配体的胞外区域，一个单一的 跨膜螺旋，以及包含细胞内必需的激酶结构域的细胞内区域 发信号。对于许多 RTK，它们的同源配体形成稳定的同二聚体，并且二聚配体与 RTK 的胞外区域驱动受体二聚化，然后使两个胞内激酶靠近 接近，使它们能够自磷酸化。磷酸化激酶可以进一步招募效应蛋白， 从而触发下游信号级联。这种“配体诱导二聚化”是长期存在的 RTK 激活模型，并已通过广泛的结构和功能得到了很好的表征 研究。尽管如此，有人建议 RTK 家族中的几个成员使用独特的激活 机制。例如，单独的 MuSK 受体不能直接与其配体结合并被其激活 Agrin，但需要肌肉细胞表面的辅助受体 Lrp4 的帮助才能激活。此外， TAM受体的激活不仅需要其配体的结合，还需要 来自质膜的磷脂酰丝氨酸脂质 (PtdSer)。此外，我们的初步结构结果 研究表明，与所有其他 RTK 不同，一个 HGF 分子可以同时接合两个 c-MET 受体 通过利用两个不同的界面；因此，单个 HGF 足以激活 c-MET 受体， 这代表了 RTK 激活机制的另一个范例。该项目的目标是研究 几种特殊RTK的结构和功能，包括MuSK、TAM和c-MET受体，其 激活机制仍知之甚少。解决不同独特的高分辨率结构 处于配体结合活性状态的 RTK 家族成员将解释区分这些的具体特征 来自其他 RTK 的受体，并揭示 RTK 激活的共同机制和多样性。 目标 1 将重点关注 MuSK 受体复合物的功能和结构分析。这项研究将 揭示 MuSK、Lrp4 和 Agrin 之间的详细结合模式，并解释为什么辅助受体 Lrp4 至关重要 用于激活 MuSK。目标 2 将重点关注 TAM 受体的生化和结构分析 膜相关的功能状态。这项研究的结果将使我们能够解释功能 PtdSer 在 TAM 激活中的重要性。目标 3 将重点关注 c-MET 的结构测定 处于 HGF 结合活性状态的受体，以了解为什么单个 HGF 分子足以用于 c-MET 激活。

Receptor tyrosine kinases (RTKs) play key roles in regulating normal cellular processes and are linked to many human diseases. Each RTK protomer contains an extracellular region that binds activating ligands, a single transmembrane helix, and an intracellular region that contains the kinase domain necessary for intracellular signaling. For many RTKs, their cognate ligands form stable homodimers, and the binding of dimeric ligand to the extracellular region of RTK drives receptor dimerization, which then brings two intracellular kinases in close proximity, enabling their autophosphorylation. The phosphorylated kinases can further recruit effector proteins, and thereby triggering downstream signaling cascade. This “ligand-induced-dimerization” is the long-standing model for the activation of RTKs, and has been well characterized by extensive structural and functional studies. Nevertheless, it has been suggested that several members in RTK family use unique activation mechanisms. For instance, the MuSK receptor alone cannot directly bind to and be activated by its ligand Agrin, but requiring assistance of the co-receptor Lrp4 on the muscle cell surface for activation. In addition, the activation of TAM receptor requires not only the binding of its ligands, but also the involvement of phosphatidylserine lipid (PtdSer) from plasma membrane. Furthermore, our preliminary structure results showed that, different to all other RTKs, one HGF molecular can simultaneously engages two c-MET receptors by utilizing two distinct interfaces; therefore, a single HGF is sufficient for the activation of c-MET receptor, which represents another paradigm in activation mechanisms of RTK. The goal of this project is to study the structures and functions of several special RTKs, including MuSK, TAM and c-MET receptors, whose activation mechanism are still poorly understood. Solving the high-resolution structures of different unique members of RTK family in the ligand-bound active state will explain the specific features that differentiate these receptors from other RTKs, and reveal the common mechanism and diversification in the activation of RTK. Aim 1 will be focused on the functional and structural analyses of MuSK receptor complex. This study will reveal the detailed binding mode between MuSK, Lrp4 and Agrin, and explain why co-receptor Lrp4 is critical for the activation of MuSK. Aim 2 will be focused on biochemical and structural analyses of TAM receptor in the membrane associated functional state. The result from this study will allow us to explain the functional importance of PtdSer in TAM activation. Aim 3 will be focused on the structural determination of c-MET receptor in the HGF bound active state to understand why single HGF molecular is sufficient for c-MET activation.