Shapiro M J, Coughlin S R
Cardiovascular Research Institute, University of California, San Francisco, California 94143-0130, USA.
J Biol Chem. 1998 Oct 30;273(44):29009-14. doi: 10.1074/jbc.273.44.29009.
Protease-activated receptor 1 (PAR1), a G protein-coupled, protease-activated receptor for the serine protease thrombin, is activated when thrombin cleaves its amino-terminal exodomain. This irreversible mechanism of activation may have necessitated an unusual pattern of receptor trafficking. Unactivated PAR1 cycles tonically between the cell surface and an intracellular pool, providing an intracellular store of uncleaved receptors and allowing repopulation of the surface with uncleaved receptors after thrombin exposure without new receptor synthesis. Activated PAR1 internalizes rapidly and is degraded in lysosomes. We report characterization of a PAR1 mutant that trafficked like the wild-type receptor when activated but did not internalize and recycle in the absence of agonist. This complements a previous study in which a mutant with normal tonic internalization but defective agonist-triggered internalization was described. These observations suggest that the trafficking behaviors of unactivated and activated PAR1 are specified by distinct signals within the receptor and imply that PAR1 internalization in the presence or absence of agonist may be mediated by distinct molecular machinery. PAR1 mutants that did not internalize in the absence of agonist were also shown to localize exclusively to the cell surface and to be defective in their ability to repopulate the cell surface with uncleaved receptors after thrombin exposure. These observations suggest that tonic internalization is necessary for maintenance of the intracellular PAR1 pool.
蛋白酶激活受体1（PAR1）是一种与G蛋白偶联的、针对丝氨酸蛋白酶凝血酶的蛋白酶激活受体，当凝血酶切割其氨基末端胞外域时被激活。这种不可逆的激活机制可能需要一种不同寻常的受体转运模式。未激活的PAR1在细胞表面和细胞内池之间持续循环，提供未切割受体的细胞内储存库，并在凝血酶暴露后允许未切割的受体重新填充到细胞表面，而无需新的受体合成。激活的PAR1迅速内化并在溶酶体中降解。我们报告了一种PAR1突变体的特征，该突变体在激活时的转运方式与野生型受体相似，但在没有激动剂的情况下不会内化和再循环。这补充了之前的一项研究，该研究描述了一种具有正常持续内化但激动剂触发内化缺陷的突变体。这些观察结果表明，未激活和激活的PAR1的转运行为由受体内不同的信号决定，这意味着在有或没有激动剂的情况下PAR1的内化可能由不同的分子机制介导。在没有激动剂的情况下不会内化的PAR1突变体也被证明仅定位于细胞表面，并且在凝血酶暴露后用未切割的受体重新填充细胞表面的能力存在缺陷。这些观察结果表明，持续内化对于维持细胞内PAR1库是必要的。
