Lipocalins are a family of proteins characterized by a conserved eight-stranded β-barrel structure with a ligand binding pocket. They perform a wide range of biological functions and this functional multiplicity must relate to the lipid partner involved. Apolipoprotein D (ApoD) and its insect homologues, Lazarillo (Laz) and Neural Lazarillo (NLaz), share common ancestral functions like longevity, stress resistance and lipid metabolism regulation, coexisting with very specialized functions, like courtship behavior. Using tryptophan fluorescence titration we screened binding of fifteen potential lipid partners for NLaz, ApoD and Laz and uncovered several novel ligands with apparent dissociation constants in the low micromolar range. Retinoic acid (RA), retinol, fatty acids and sphingomyelin are shared ligands. Sterols however showed a species specific binding pattern: Cholesterol did not show strong binding to human ApoD, while NLaz and Laz did bind ergosterol. Among the Lipocalin-specific ligands, we found that ApoD selectively binds the endocannabinoid anandamide but not 2-acylglycerol, and that NLaz binds the pheromone 7-tricosene but not 7,11-heptacosadiene or 11-cis-vaccenyl acetate. To test the functional relevance of Lipocalin ligand-binding at the cellular level, we analyzed the effect of ApoD, Laz and NLaz preloaded with RA on neuronal differentiation. Our results show that ApoD is necessary and sufficient to allow for RA differentiating activity. Both human ApoD and Drosophila NLaz successfully deliver RA to immature neurons, driving neurite outgrowth. We conclude that ApoD, NLaz and Laz bind selectively to a different but overlapping set of lipid ligands. This multispecificity can explain their varied physiological functions. This article is protected by copyright. All rights reserved.
This article is protected by copyright. All rights reserved.

PMID:

 

23777559

 

[PubMed – as supplied by publisher]