Step 1: Amino acid single letter codes are standardized: Lysine = K, Aspartic acid = D, Glutamic acid = E, Leucine = L.
Step 2: Substituting each three letter abbreviation in the given order Lys-Asp-Glu-Leu with its single letter equivalent produces the string K, D, E, L in sequence.
Step 3: This four letter motif KDEL is biologically significant, it is the classic retrieval signal recognized by KDEL receptors in the Golgi that send escaped ER-resident proteins back to the endoplasmic reticulum.
Step 4: The resulting code is \[\boxed{\text{KDEL}}\]