KDEL is the sorting signal for luminal ER proteins. In this a classic experiment for day 256 of #LMB365, lysozyme was expressed in COS cells without (left) or with (right) the addition of the sequence SEKDEL, and detected by immunofluorescence. Normally, lysozyme is rapidly secreted and the main pool of intracellular protein is in transit through the Golgi apparatus. Addition of KDEL causes it to accumulate to high levels in the ER. This was the original proof that (SE)KDEL is a sufficient signal for ER retention.
MRC Laboratory of Molecular Biology
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