The nonspecific lipid transfer protein sterol carrier protein 2 (SCP2) is involved in organellar fatty acid metabolism. A hydrophobic cavity in the structure of SCP2 accommodates a wide variety of apolar ligands such as cholesterol derivatives or fatty acyl-coenzyme A (CoA) conjugates. The properties of this nonspecific lipid binding pocket are explored using NMR chemical shift perturbations, paramagnetic relaxation enhancement, amide hydrogen exchange, and 15 N relaxation measurements. A common binding cavity shared by different physiological ligands is identified. NMR relaxation measurements reveal that residues in the three C-terminal α-helices within the lipid binding region exhibit mobility at fast (picosecond to nanosecond) and slow (microsecond to millisecond) time scales. Ligand binding is associated with a considerable loss of peptide backbone mobility. The observed conformational dynamics in SCP2 may play a role for the access of hydrophobic ligands to an occluded binding pocket. The C-terminal peroxisomal targeting signal of SCP2 is specifically recognized by the Pex5p receptor protein, which conducts cargo proteins toward the peroxisomal organelle. Neither the C-terminal targeting signal nor the N-terminal precursor sequence interferes with lipid binding by SCP2. The α-helices involved in lipid binding also mediate a secondary interaction interface with the Pex5p receptor. Silencing of conformational dynamics of the peptide backbone in these helices upon either lipid or Pex5p binding might communicate the loading state of the cargo protein to the targeting receptor.
Graphical Abstract
HHS Public AccessAuthor manuscript Biochemistry. Author manuscript; available in PMC 2016 September 08.
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Author ManuscriptLipid transfer proteins are auxiliary shuttles ensuring the appropriate localization of lipids in cellular subcompartments. Sterol carrier protein 2 (SCP2 1 ) has been reported to interact with a wide variety of lipids associated with peroxisomal metabolism, including acyl-CoA conjugates as high-affinity ligands (1-3). Disruption of the scp2 gene in mice suggests bile acids (soluble derivatives of cholesterol) and branched-chain fatty acid CoAs as physiological ligands to be functionally associated with SCP2 (4-6). Enzymes involved in both cholesterol secretion and fatty acid oxidation include the peroxisomal thiolase, acylCoA hydrogenase, and enoyl-CoA hydratase (7,8), all of which are expressed as fusion proteins with a sterol carrier protein domain fold (9). The transfer of fatty acyl-CoA substrates by SCP2 may have evolved to assist enzymes involved in branched chain fatty acid α-oxidation or very long chain fatty acid β-oxidation in the peroxisome (8). In this context SCP2 may play a role in substrate and/or product binding during peroxisomal bile acid synthesis and fatty acid oxidation.Some molecular insight into the ligand binding site of the sterol carrier protein fold has been obtained from structural high-resolution studies of SC...