Hepatic Fatty Acid-Binding Proteins of a Teleost, Lateolabrax japonicus. The Primary Structures and Location of a Disulfide Bond

Two fatty acid-binding proteins (FABP), FABP-1 and FABP-2, were purified from the liver cytosol of the teleost, Lateolabrax japonicus (Japan sea bass), and characterized. The complete primary structure of FABP-2 was determined by protein analysis to be the following: MDFSGTWQVY AQENYEEFLR AMELPADVIK MAKDIKPITE IKQSGNDFVV TSKTPGKTVT NSFTIGKEAD ITTMDGKKIR CVVNLEGGKL VCNTGKFCHI QELRGGEMVE TLTMGSTTLI RKSKKM. Partial peptide sequences of FABP-1 were also determined. Phylogenetic analysis indicates that FABP-2 is a homologue of mammalian hepatic FABP, whereas FABP-1 is most similar to the members of mammalian cardiac FABP subfamily. L. japonicus FABP-2 contains three cysteine residues, and a disulfide bond is identified between Cys-81 and Cys-92. A theoretical model of FABP-2 generated by a homology modeling method indicates close proximity of the two cysteine residues in the three-dimensional structure. This is a rather rare case of cytosolic protein having a disulfide bond under the normally reducing conditions of the cytosol, though the presence or absence of disulfide bonds does not seem to affect the ligand-binding ability.