Identification of the bile acid-binding site of the ileal lipid-binding protein by photoaffinity labeling, matrix-assisted laser desorption ionization-mass spectrometry, and NMR structure
W. Kramer et al., Identification of the bile acid-binding site of the ileal lipid-binding protein by photoaffinity labeling, matrix-assisted laser desorption ionization-mass spectrometry, and NMR structure, J BIOL CHEM, 276(10), 2001, pp. 7291-7301
The ileal lipid-binding protein (ILBP) is the only physiologically relevant
bile acid-binding protein in the cytosol of ileocytes, To identify the bil
e acid-binding site(s) of ILBP, recombinant rabbit ILBP photolabeled with 3
-azi- and 7-azi-derivatives of cholyltaurine was analyzed by a combination
of enzymatic fragmentation, gel electrophoresis, and matrix-assisted laser
desorption ionization (MALDI)-mass spectrometry, The attachment site of the
3-position of cholyltaurine was localized to the amino acid triplet His(10
0)-Thr(101)-Ser(102) using the photoreactive 3,3-azo-derivative of cholylta
urine, With the corresponding 7,7-azo-derivative, the attachment point of t
he 7-position could be localized to the C-terminal part (position 112-128)
as well as to the N-terminal part suggesting more than one binding site for
bile acids. By chemical modification and MMR structure of ILBP, arginine r
esidue 122 was identified as the probable contact point for the negatively
charged side chain of cholyltaurine, Consequently, bile acids bind to ILBP
with the steroid nucleus deep inside the protein cavity and the negatively
charged side chain near the entry portal, The combination of photoaffinity
labeling, enzymatic fragmentation, MALDI-mass spectrometry, and NMR structu
re was successfully used to determine the topology of bile acid binding to
ILBP.