MOLECULAR-CLONING, TISSUE DISTRIBUTION, AND EXPRESSION OF A 14-KDA BILE ACID-BINDING PROTEIN FROM RAT ILEAL CYTOSOL

A cDNA clone encoding the major intestinal cytosolic 14 kDa bile acid- binding protein (14-kDa I-BABP) was isolated from a rat heal lambda gt 22A library following immunoscreening using a monospecific antiserum r aised against a 14 kDa polypeptide found in the rat ileal cytosol. One clone of 516 bp encoded a 128-amino acid protein with a predicted mol ecular mass of 14,544 Da. The deduced amino acid sequence of 14 kDa I- BABP showed 100% homology to rat intestinal 15-kDa protein (I-15P) and 72% homology to porcine 15-kDa gastrotropin, whereas comparison of I- BABP to rat 14 kDa fatty acid-binding proteins of liver, intestine, an d heart revealed homologies of 44%, 25%, and 28%, respectively. Northe rn blot analysis revealed a single transcript of approximate to 0.5 kb in ileum and ovary; however, the abundance of I-BABP mRNA was much gr eater in ileum than in ovary. No transcript was seen in RNA extracted from stomach, jejunum, colon, liver, adrenal, brain, heart, kidney, or testis. Transfection of the I-BABP cDNA into COS-7 cells resulted in the expression of a 14-kDa protein that was identical to the deal cyto solic I-BABP as determined by immunoblotting. Photoaffinity labeling o f expressed 14-kDa protein was saturable with respect to increasing co ncentrations of 7,7-azo[H-3]taurocholate (K-m, 83.3 mu M; V-max, 6.7 p mol/mg per 5 min). Taurocholate inhibited 7,7-azotaurocholate labeling by >96% with lesser inhibition by taurochenodeoxycholate (83.1%), che nodeoxycholate (74.6%), cholate (50.5%), and progesterone (38.5%), whe reas oleic acid and estradiol did not inhibit binding.