KDEL is a target peptide sequence in the amino acid structure of a protein which prevents the protein from being secreted from the endoplasmic reticulum (ER). A protein with a functional KDEL motif will be retrieved from the Golgi apparatus by retrograde transport to the ER lumen. It also targets proteins from other locations (such as the cytoplasm) to the ER. Proteins can only leave the ER after this sequence has been cleaved off.
The abbreviation KDEL is formed by the corresponding letters to each amino acid. This letter system was defined by the IUPAC and IUBMB in 1983, and is as follows:
Therefore, the sequence in three letter code is: Lys-Asp-Glu-Leu.
The soluble resident protein will remain in the ER as long as it contains a KDEL signal sequence. However, since vesicle budding is such a dynamic process, and there is a high concentration of soluble proteins in the ER, soluble proteins are inadvertently transported to the cis-golgi via COP2 coated vesicles. Upon fusion with the cis-golgi the soluble protein is released where its KDEL signal sequence is recognised by receptors located on the membrane. The receptor and soluble protein are transported via COP1 coated vesicles back to the ER where the contents of the vesicle are released and the soluble protein dissociates from the receptor. The association of the protein in the cis-golgi and dissociation in the ER is mediated by pH, since a slightly acidic pH, found in the cis-golgi provides optimum binding conditions.