Developmental and structural studies of an intracellular lipid binding protein expressed in the ileal epithelium

Enterocytes located in the pig distal small intestine (ileum) contain a cytosolic protein that is homologous to two proteins that are also synthesized in these cells: intestinal and "liver" fatty acid-binding proteins (I- and L-FABPc, respectively). To begin to investigate the functional interrelationships of these three proteins, we compared their patterns of tissue-specific expression and developmental regulation in the mouse. Blot hybridization analyses of RNA prepared from 12 adult tissues revealed that this mRNA was confined to the small intestine. Unlike I- and L-FABPc mRNA, which are most abundant in the proximal jejunum, this mRNA is most abundant in the ileum. While I- and L-FABPc gene transcription commence in late fetal life coincident with initial cytodifferentiation of the mouse gut epithelium, the ileal gene is activated later, at the suckling/weaning transition (postnatal day 12). The ileal location and developmental pattern of expression suggested that this protein may play a role in the intracellular transport of bile salts in the ileal epithelium. To test this hypothesis, we expressed the porcine ileal peptide (PIP) in Escherichia coli, purified it to apparent homogeneity, and analyzed its binding properties for bile acids and fatty acids using 13C NMR spectroscopy. Like I-FABPc, PIP binds palmitate and oleate with a 1:1 molar stoichiometry. However, unlike I-FABPc PIP binds chenodeoxycholate. In addition, the presence of chenodeoxycholate blocks fatty acid binding to PIP, but not to I-FABPc. E. coli-derived PIP was subsequently crystallized with and without chenodeoxycholic acid. All crystals are orthorhombic in the P2(1)2(1)2(1) space group. The unit cell dimensions are a = 36.15 A, b = 50.13 A, and c = 67.18 A.