Image-based and biochemical assays to investigate endosomal protein sorting.

The sorting of membrane proteins within the endosomal system occurs through a panoply of highly dynamic sequential molecular interactions that together govern many physiologically important processes. A key component of the endosomal protein sorting machinery is the retromer complex. Through two distinct subcomplexes, retromer operates to select cargo for endosome-to-Golgi retrieval and also drives membrane tubule formation. Many accessory proteins associate with retromer to facilitate protein sorting and/or tubule formation. The experience we have gained from studying retromer-mediated endosomal protein sorting and the assays developed and applied in the course of these studies can provide a template for researchers interested in related endosomal trafficking pathways. Herein we describe image-based assays that can be applied to study endosomal protein sorting through the use of antibody-uptake assays in low-, medium-, and high-throughput formats. We additionally detail simple but effective native immunoprecipitation methods that can be employed to identify novel proteins that may interact transiently with a protein of interest within the endosomal pathway.