Cells communicate with their environment via surface proteins and secreted factors. Unconventional protein secretion (UPS) is an evolutionarily conserved process, via which distinct cargo proteins are secreted upon stress. Most UPS types depend upon the Golgi-associated GRASP55 protein. However, its regulation and biological role remain poorly understood. Here, we show that the mechanistic target of rapamycin complex 1 (mTORC1) directly phosphorylates GRASP55 to maintain its Golgi localization, thus revealing a physiological role for mTORC1 at this organelle. Stimuli that inhibit mTORC1 cause GRASP55 dephosphorylation and relocalization to UPS compartments. Through multiple, unbiased, proteomic analyses, we identify numerous cargoes that follow this unconventional secretory route to reshape the cellular secretome and surfactome. Using MMP2 secretion as a proxy for UPS, we provide important insights on its regulation and physiological role. Collectively, our findings reveal the mTORC1-GRASP55 signaling hub as the integration point in stress signaling upstream of UPS and as a key coordinator of the cellular adaptation to stress.

细胞通过表面蛋白和分泌因子与其环境交流。非常规蛋白质分泌 (UPS) 是一个进化上保守的过程，通过该过程可以在压力下分泌不同的货物蛋白。大多数 UPS 类型依赖于高尔基体相关的 GRASP55 蛋白。然而，它的调节和生物学作用仍然知之甚少。在这里，我们表明雷帕霉素复合物 1 (mTORC1) 的机制靶点直接磷酸化 GRASP55 以维持其高尔基体定位，从而揭示 mTORC1 在该细胞器中的生理作用。抑制 mTORC1 的刺激会导致 GRASP55 去磷酸化并重新定位到 UPS 隔间。通过多次、公正的蛋白质组学分析，我们确定了许多遵循这种非常规分泌途径来重塑细胞分泌组和表面组的货物。使用 MMP2 分泌作为 UPS 的代理，我们提供了关于其调节和生理作用的重要见解。总的来说，我们的研究结果揭示了 mTORC1-GRASP55 信号中枢是 UPS 上游压力信号的整合点，也是细胞适应压力的关键协调者。