Laboratory of Biochemical Neuroendocrinology, Clinical Research Institute of Montreal, Montreal, Quebec H2W 1R7, Canada.
J Biol Chem. 2011 Jan 28;286(4):2728-38. doi: 10.1074/jbc.M110.192344. Epub 2010 Nov 12.
The zymogen activation mechanism and physiological functions of the most ancient and highly conserved basic amino acid-specific proprotein convertase 7 (PC7) are not known. Herein, we characterized the biosynthesis, subcellular localization, and trafficking of the membrane-bound full-length rat and human PC7. The prosegment of PC7 is primarily secreted alone as a non-inhibitory protein via the conventional, Golgi-dependent, secretory pathway. Mature PC7 is partially sulfated and thus reaches the cell surface via the conventional route. However, a fraction of PC7 reaches the cell surface through a brefeldin A- and COPII-independent unconventional secretory pathway. The latter trafficking may explain the rapid (<10 min) transit of a fraction of PC7 from the ER to the cell surface. Electron microscopy further confirmed the localization of PC7 to the cell surface of HEK293 cells. Within the cytosolic tail, only two cysteines (Cys(699) and Cys(704)) are palmitoylated, but this modification does not affect the choice of trafficking pathway. Swapping the transmembrane-cytosolic tail (TMCT) sequences of the convertases Furin and PC7 revealed that PC7(TMCT-Furin) is much more sulfated and hence traffics more efficiently through the conventional secretory pathway. In contrast, the Furin(TMCT-PC7) is no longer sulfated and thus reaches the cell surface by the unconventional pathway. Because trafficking of PC7(CT-Furin) and Furin(CT-PC7) resemble their wild type counterparts, we deduce that the transmembrane domain of PC7 regulates the sorting of PC7 toward the unconventional secretory pathway. In conclusion, PC7 is distinct from other proprotein convertases in its zymogen activation, subcellular localization, and trafficking.
PC7 是最古老且高度保守的碱性氨基酸特异性蛋白原转化酶 7，其酶原激活机制和生理功能尚不清楚。本文对膜结合全长大鼠和人 PC7 的生物合成、亚细胞定位和运输进行了研究。PC7 的前肽主要通过传统的、依赖高尔基体的分泌途径作为非抑制性蛋白单独分泌。成熟的 PC7 部分硫酸化，因此通过传统途径到达细胞表面。然而，一部分 PC7 通过布雷菲德菌素 A 和 COPII 独立的非典型分泌途径到达细胞表面。这种非常规的分泌途径可能解释了一部分 PC7 从内质网快速(<10 分钟)转运到细胞表面的原因。电子显微镜进一步证实了 PC7 定位于 HEK293 细胞的细胞表面。在胞质尾部，只有两个半胱氨酸(Cys(699)和 Cys(704))被棕榈酰化，但这种修饰并不影响运输途径的选择。交换转化酶 Furin 和 PC7 的跨膜胞质尾(tmct)序列，发现 PC7(tmct-furin)被硫酸化更多，因此更有效地通过传统的分泌途径运输。相比之下，furin(tmct-pc7)不再被硫酸化，因此通过非典型途径到达细胞表面。由于 PC7(CT-Furin)和 Furin(CT-PC7)的运输类似于它们的野生型对应物，我们推断 PC7 的跨膜域调节 PC7 向非典型分泌途径的分选。总之，PC7 在酶原激活、亚细胞定位和运输方面与其他蛋白原转化酶不同。
