Identification, retinoid binding, and x-ray analysis of a human retinol-binding protein

Two cellular retinol-binding proteins (CRBP I and II) with distinct tissue distributions and retinoid-binding properties have been recognized thus far in mammals. Here, we report the identification of a human retinol-binding protein resembling type I (55.6% identity) and type II (49.6% identity) CRB Ps, but with a unique H residue in the retinoid-binding site and a distinct ively different tissue distribution. Additionally, this binding protein (CR BP III) exhibits a remarkable sequence identity (62.2%) with the recently i dentified c-crystallin/CRBP of the diurnal gecko Lygodactylus picturatus [W erten, P. J. L., Roll, B., van Alten, D. M. F. & de Jong, W. W. (2000) Proc . Natl. Acad. Sci. USA 97, 3282-3287 (First Published March 21, 2000; 10.10 73/pnas.050500597)]. CRBP III and all-trans-retinol form a complex (K-d app roximate to 60 nM), the absorption spectrum of which is characterized by th e peculiar fine structure typical of the spectra of holo-CRBP I and II. As revealed by a 2.3-Angstrom x-ray molecular model of apo-CRBP III, the amino acid residues that line the retinol-binding site in CRBP I and II are posi tioned nearly identically in the structure of CRBP III. At variance with th e human CRBP I and II mRNAs, which are most abundant in ovary and intestine , respectively, the CRBP III mRNA is expressed at the highest levels in kid ney and liver thus suggesting a prominent role for human CRBP III as an int racellular mediator of retinol metabolism in these tissues.