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The interrelationship between ligand binding and self-association of the folate binding protein. The role of detergent–tryptophan interaction

2011; Elsevier BV; Volume: 1810; Issue: 12 Linguagem: Inglês

10.1016/j.bbagen.2011.07.005

1872-8006

Jan Holm, Christian Schou, Linnéa Nygren Babol, Anders J. Lawaetz, Susanne W. Bruun, Morten Z. Hansen, Steen Ingemann Hansen,

Protein Structure and Dynamics

The folate binding protein (FBP) regulates homeostasis and intracellular trafficking of folic acid, a vitamin of decisive importance in cell division and growth. We analyzed whether interrelationship between ligand binding and self-association of FBP plays a significant role in the physiology of folate binding.Self-association behavior of apo- and holo-FBP was addressed through size exclusion chromatography, SDS-PAGE, mass spectrometry, surface plasmon resonance and fluorescence spectroscopy.Especially holo-FBP exhibits concentration-dependent self-association at pH 7.4 (pI), and is more prone to associate into stable complexes than apo-FBP. Even more pronounced was the tendency to complexation between apo-FBP and holo-FBP in accord with a model predicting association between apo and holo monomers [19]. This will lead to removal of apo monomers from the reaction scheme resulting in a weak incomplete ligand binding similar to that observed at FBP concentrations <10nM. The presence of synthetic and natural detergents normalized folate binding kinetics and resulted in appearance of monomeric holo-FBP. Fluorescence spectroscopy indicated molecular interactions between detergent and tryptophan residues located in hydrophobic structures of apo-FBP which may participate in protein associations.Self-association into multimers may protect binding sites, and in case of holo-FBP even folate from biological degradation. High-affinity folate binding in body secretions, typically containing 1-10nM FBP, requires the presence of natural detergents, i.e. cholesterol and phospholipids, to avoid complexation between apo- and holo-FBP.