Institute of Physiological Chemistry and Pathobiochemistry and Cells-in-Motion Cluster of Excellence (EXC1003-CiM), University of Münster, D-48149 Münster, Germany.
Center for Medical Biotechnology, University of Duisburg-Essen, D-45117 Essen, Germany.
J Cell Sci. 2017 Oct 1;130(19):3261-3271. doi: 10.1242/jcs.205872. Epub 2017 Aug 4.
Proteolytic processing of cell-surface-bound ligands, called shedding, is a fundamental system to control cell-cell signaling. Yet, our understanding of how shedding is regulated is still incomplete. One way to increase the processing of dual-lipidated membrane-associated Sonic hedgehog (Shh) is to increase the density of substrate and sheddase. This releases and also activates Shh by the removal of lipidated inhibitory N-terminal peptides from Shh receptor binding sites. Shh release and activation is enhanced by Scube2 [signal sequence, cubulin (CUB) domain, epidermal growth factor (EGF)-like protein 2], raising the question of how this is achieved. Here, we show that Scube2 EGF domains are responsible for specific proteolysis of the inhibitory Shh N-terminus, and that CUB domains complete the process by reversing steric masking of this peptide. Steric masking, in turn, depends on Ca occupancy of Shh ectodomains, unveiling a new mode of shedding regulation at the substrate level. Importantly, Scube2 uncouples processing of Shh peptides from their lipid-mediated juxtamembrane positioning, and thereby explains the long-standing conundrum that N-terminally unlipidated Shh shows patterning activity in Scube2-expressing vertebrates, but not in invertebrates that lack Scube orthologs.
细胞表面结合配体的蛋白水解加工，称为脱落，是控制细胞间信号的基本系统。然而，我们对脱落如何被调控的理解仍然不完整。增加双脂化膜相关 Sonic hedgehog（Shh）的加工的一种方法是增加底物和脱落酶的密度。这通过从 Shh 受体结合位点去除脂化的抑制性 N 端肽来释放和激活 Shh。Shh 的释放和激活被 Scube2[信号序列、cubulin（CUB）结构域、表皮生长因子（EGF）样蛋白 2]增强，这引发了一个问题，即这是如何实现的。在这里，我们表明 Scube2 的 EGF 结构域负责特异性切割抑制性 Shh N 端，而 CUB 结构域通过反转该肽的空间位阻掩蔽来完成该过程。空间位阻掩蔽反过来又取决于 Shh 外域的 Ca 占据，揭示了一种新的底物水平脱落调控模式。重要的是，Scube2 将 Shh 肽的加工与其脂介导的近膜定位解耦，从而解释了长期存在的难题，即 N 端未脂化的 Shh 在表达 Scube2 的脊椎动物中具有图案形成活性，但在缺乏 Scube 同源物的无脊椎动物中没有。
