Structural studies of a baboon (Papio sp.) plasma protein inhibitor of cholesteryl ester transferase

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. R elative to CETIP, baboon apoC-I is a weak inhibitor of baboon cholesteryl e ster transferase (CET). To study the structural features responsible for CE T inhibition, CETIP was synthesized by solid-phase methods. Using sodium do decyl sulfate (SDS) to model the lipoprotein environment, the solution stru cture of CETIP was probed by optical and H-1 NMR spectroscopy, Circular dic hroism 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 no t buried deeply in the lipid environment. The helical nature of CETIP in th e presence of SDS was confirmed by upfield H-1(alpha) secondary shifts and an average solution structure determined by distance geometry/simulated ann ealing calculations using 476 NOE-based distance restraints. The backbone ( N-C-alpha-C=O) root-mean-square deviation of an ensemble of 17 out of 25 ca lculated structures superimposed on the average structure was 1.06 +/- 0.30 Angstrom using residues V4-P35 and 0.51 +/- 0.17 Angstrom using residues A 7-S32, Although the side-chain orientations fit the basic description of a class A amphipathic helix, both intramolecular salt bridge formation and "s norkeling" of basic side chains toward the polar face play minor, if any, r oles in stabilizing the lipid-bound amphipathic structure. Conformational f eatures of the calculated structures for CETIP are discussed relative to mo dels of CETIP inhibition of cholesteryl ester transferase.