Abstract
While N-glycans are synthesized in the lumens, some of them reach the cytosolic side of membranes through retro-translocation independent of endoplasmic-reticulum-associated degradation. Here, we present a protocol to measure the topology of N-glycans in a transmembrane protein, based on the principle that cytosolic but not luminal N-glycans are trimmed by PNGase F in the absence of detergent. We describe the procedures for this protocol consisting of microsome preparation from cells, PNGase F accessibility assay, and western blot analysis. For complete details on the use and execution of this protocol, please refer to Wang et al.1.