Quantification of Protein Exit at the Trans-Golgi Network

With one-third of all newly synthesized proteins entering the secretory pathway, correct protein sorting is essential for cellular homeostasis. In the last three decades, researchers have developed numerous biochemical, genetic, and cell biological approaches to study protein export and sorting from the trans-Golgi network (TGN). However, accurately quantifying protein transport from one compartment to the next in the secretory pathway has been challenging. The Retention Using Selective Hooks (RUSH) system is a method that allows monitoring trafficking of a protein of interest in real time, similar to a pulse-chase experiment but without the need of radiolabeling. Accurate calculations, however, are necessary and currently lacking. Here, we combine the RUSH system with live cell imaging to quantify and calculate half lives. We exemplify our approach using a soluble secreted protein (LyzC). This system will benefit membrane trafficking researchers by adding numbers to protein export and comparing the export kinetics of different cargoes and variating conditions.