Molecular cloning, expression, and characterization of a human intestinal 15-kDa protein

We have isolated a cDNA encoding a human intestinal 15-kDa protein (I-15P) from a human ileal lambda gt 11 cDNA library, using a full-length rat I-15P cDNA. One clone encompassed 571 nucleotides and encoded a 128-amino-acid protein with a calculated molecular mass of 14355 Da. The deduced amino acid sequence of human I-15P showed high similarity to the rat counterpart (78%), mouse ileal lipid-binding protein (80%) and porcine gastrotropin (75%). It also exhibited 36% similarity to human liver fatty-acid-binding protein (L-FABP). Northern blot analysis of human I-15P revealed a single transcript only in ileum, however, the reverse-transcription/PCR demonstrated expression in ovary and placenta, but it was much lower than in ileum. Transformation of Escherichia coli with the I-15P cDNA resulted in the efficient expression of a protein that was identical to the ileal cytosolic I-15P. In vitro binding studies revealed that the bacterially expressed recombinant I-15P showed much lower affinities for palmitate and oleate than L-FABP. However, it showed similar affinity for taurocholate, compared with a control, BSA. Comparison of the structural features of human I-15P and human L-FABP suggested that loss of a long alpha-helix region and hydrophobic profile of I-15P may be attributable to a unique ligand-binding specificity of I-15P.