KKXX and for some proteins XKXX is a target peptide motif located in the C terminus in the amino acid structure of a protein responsible for retrieval of endoplasmic reticulum (ER) membrane proteins from the Golgi apparatus. The signal interacts with the COPI protein complex to target the ER from the cis end of the Golgi apparatus by retrograde transport.
The abbreviation KKXX is formed by the corresponding standard abbreviations for lysine (K) and any amino acid (X). This letter system was defined by the IUPAC and IUBMB in 1983, and is as follows:
References
KKXX (amino acid sequence) Wikipedia(Text) CC BY-SA