STRUCTURAL-CHANGES IN HUMAN TEAR LIPOCALINS ASSOCIATED WITH LIPID-BINDING

Structural and conformational changes in tear lipocalins were detected in association with ligand binding and release. Circular dichroism me asurements demonstrated that ligand binding induces beta structure for mation, aromatic side chain asymmetry, and a more rigid state in tear lipocalins (TL). The exposure of the tyrosyl component is less in apo- TL than in holo-TL. The sole tryptophan residue, Trp(17), is buried in both holo- and apo-TL. The steady state exposure of Trp(17) is the sa me in holo- and apo-TL, but the dynamic exposure is two-fold greater i n apo-TL. Maneuvers to unfold the protein with urea or incubation in a n acidic environment resulted in increased exposure of aromatic amino acids. Electron paramagnetic resonance studies verified that lipids ar e liberated from TL in an acidic environment. Acidic pH promotes confo rmational changes in TL involving aromatic residues, particularly the conserved residue Trp(17). These changes are associated with lipid rel ease. The liberation of lipid from the cavity of TL under acidic condi tions involves a molten globule state of the protein. We postulate tha t TL, exposed to the steep surface pH gradient that exists at lipid-aq ueous interfaces, would release lipid in association with a molten glo bule transition. The data suggest a plausible regulatory mechanism for lipid delivery from lipocalins at the tear film surface. (C) 1998 Els evier Science B.V. All rights reserved.