Structural and functional characterization of CATH_BRALE, the defense molecule in the ancient salmonoid, Brachymystax lenok

Thick-lipped lenok, Brachymystax lenok is one of the ancient fish species in China and northeast Asia countries. Due to the overfishing, the population of lenok has been declined significantly. Cathelicidins are innate immune effectors that possess both bactericidal activities and immunomodulatory functions. This report identifies and characterizes the salmonoid cathelicidin (CATH_BRALE) from this ancient fish. It consists of open reading frame (ORF) of 886 bp encoding the putative peptide of 199 amino acids. Sequence alignment with other representative salmonid cathelicidins displayed two distinctive features of current lenok cathelicidin: high level of arginine, resulting in high positive charge and glycine residues, which is significantly different from most acknowledged types of cathelicidins; and the six-amino-acid tandem repeated sequence of RPGGGS detected in a variable number of copies among fish cathelicidins, suggesting the existence of a genetically unstable region similar to that found in some mammalian cathelicidins. Expression of CATH_BRALE is predominantly found in gill, with lower levels in the gastrointestinal tract and spleen. The homology modeled structure of CATH_BRALE exhibits structural features of antiparallel β-sheets flanked by α-helices that are representative of small cationic cathelicidin family peptides. CATH_BRALE possesses much stronger antimicrobial activity against gram-negative bacteria than that of the human ortholog, LL-37. The growth of two typical fish bacterial pathogens, gram-negative bacterium of Aeromonas salmonicida and Aeromonas hydrophila was substantially inhibited by synthetic CATH_BRALE, with both MICs as low as 9.38 μM.