Processing of the L5–67 precursor peptide and characterization of LUQIN in the LUQ neurons of Aplysia californica

Metabolic labeling of the dorsal Left Upper Quadrant (LUQ) cells of the abdominal ganglion of Aplysia californica and RP-HPLC separation of their peptide content allowed us to identify the L5–67 precursor and its processed peptides. Cleavage of the signal peptide occurred between amino acids 23 and 24 of the prepropeptide and generated a propeptide of 89 amino acids. Further processing by endopeptidases at the twin basic residues Lys 12-Arg 13 of the precursor generated a peptide of 76 amino acids, as well as an amidated decapeptide, LUQIN. The sequence of LUQIN was determined by amino acid sequencing and by its comigration with the synthetic peptide Ala-Pro-Ser-Trp-Arg-Pro-Gln-Gly Arg-Phe-amide in three different RP-HPLC systems. The amidation of LUQIN was further demonstrated by its resistance to carboxypeptidase A digestion.