A 38-residue protein associated with cholesteryl
ester transfer inhibition has been identified in baboons
(Papio sp.). The cholesteryl ester transfer inhibitor
protein (CETIP) corresponds to the N-terminus of baboon
apoC-I. Relative to CETIP, baboon apoC-I is a weak inhibitor
of baboon cholesteryl ester transferase (CET). To study
the structural features responsible for CET inhibition,
CETIP was synthesized by solid-phase methods. Using sodium
dodecyl sulfate (SDS) to model the lipoprotein environment,
the solution structure of CETIP was probed by optical and
1H NMR spectroscopy. Circular dichroism data
show that the protein lacks a well-defined structure in
water but, upon the addition of SDS, becomes helical (56%).
A small blue shift of 8 nm was observed in the intrinsic
tryptophan fluorescence of CETIP in the presence of saturating
amounts of SDS, suggesting that tryptophan-23 is not buried
deeply in the lipid environment. The helical nature of
CETIP in the presence of SDS was confirmed by upfield
1Hα secondary shifts and an
average solution structure determined by distance geometry/simulated
annealing calculations using 476 NOE-based distance restraints. The
backbone (N-Cα-C=O) root-mean-square deviation of
an ensemble of 17 out of 25 calculated structures superimposed on
the average structure was 1.06 ± 0.30 Å using residues
V4–P35 and 0.51 ± 0.17 Å using residues A7–S32.
Although the side-chain orientations fit the basic description
of a class A amphipathic helix, both intramolecular salt
bridge formation and “snorkeling” of basic side
chains toward the polar face play minor, if any, roles in
stabilizing the lipid-bound amphipathic structure. Conformational
features of the calculated structures for CETIP are discussed
relative to models of CETIP inhibition of cholesteryl ester
transferase.