Isolation, characterization, and cDNA sequence of two fatty acid-binding proteins from the midgut of Manduca sexta larvae.

Two abundant fatty acid-binding proteins (MFB1 and MFB2) were isolated from the midgut cytosol of larval Manduca sexta. As isolated, MFB1 and MFB2 were found to contain bound fatty acids in a 1:1 molar stoichiometric ratio. Immunological screening demonstrated that MFB1 and MFB2 were restricted to the midgut in a gradient distribution, with MFB1 more concentrated in the anterior two-thirds of the midgut and MFB2 more concentrated in the posterior two-thirds of the midgut. MFB1 exchanged fatty acid more readily than did MFB2. MFB1 was about 2% and MFB2 about 12% of the cytosolic protein in the midgut. cDNA clones for MFB1 and MFB2 both encode proteins of 131 amino acids that are rich in lysine and acidic residues. Analysis of the amino acid sequence alignment of the MFBs with six mammalian fatty acid-binding proteins revealed a number of shared features: 9 conserved glycines, presumably important in turns of the beta-strands; a basic amino acid in a position corresponding to the residue reported to participate in binding the carboxyl group of the fatty acid (Arg in MFB1 and Lys in MFB2); and conservation of many of the residues important in binding the aliphatic portion of the fatty acid.