Termination of signaling by protease-activated receptor-1 is linked to lysosomal sorting

The irreversible proteolytic mechanism by which protease-activated receptor-1 (PAR1), the G protein-coupled receptor (GPCR) for thrombin, is activated raises the question of how it is shut off. Like classic GPCRs, activated PAR1 is rapidly phosphorylated and internalized, but unlike classic GPCRs, which recycle, internalized PAR1 is sorted to lysosomes. A chimeric PAR1 bearing the substance P receptor’s cytoplasmic carboxyl tail sequestered and recycled like wild-type substance P receptor. In cells expressing this chimera, signaling in response to the PAR1-activating peptide SFLLRN ceased as expected upon removal of this agonist. Strikingly, however, when the chimera was activated proteolytically by thrombin, signaling persisted even after thrombin was removed. This persistent signaling was apparently due to “resignaling” by previously activated receptors that had internalized and recycled back to the cell surface. Thus the cytoplasmic carboxyl tail of PAR1 specifies an intracellular sorting pattern that is linked to its signaling properties. In striking contrast to most GPCRs, sorting of activated PAR1 to lysosomes rather than recycling is critical for terminating PAR1 signaling—a trafficking solution to a signaling problem.

蛋白酶激活受体1（protease-activated receptor-1, PAR1）作为凝血酶的G蛋白偶联受体（G protein-coupled receptor, GPCR），其通过不可逆蛋白水解通路完成激活的机制，引发了一个关键科学问题：该受体的信号转导如何被终止。与经典GPCR类似，激活后的PAR1会被快速磷酸化并内吞；但与可循环回收的经典GPCR不同，内吞后的PAR1会被分选至溶酶体。携带P物质受体胞质羧基末端的嵌合型PAR1，会像野生型P物质受体一样被截留并循环回收。在表达该嵌合受体的细胞中，当移除PAR1激活肽SFLLRN的激动剂后，其响应该肽的信号转导会如预期般终止。但令人意外的是，当该嵌合受体被凝血酶以蛋白水解方式激活后，即便移除了凝血酶，其信号转导仍会持续存在。这种持续的信号转导显然是由先前已激活、经内吞后循环返回细胞表面的受体所介导的“复信号转导”（resignaling）所致。由此可见，PAR1的胞质羧基末端决定了其细胞内分选模式，而该模式与其信号转导特性直接相关。与绝大多数GPCR形成鲜明对比的是，激活后的PAR1被分选至溶酶体而非循环回收通路，这对于终止PAR1的信号转导至关重要——这是一种通过膜泡运输策略解决信号转导终止问题的独特机制。