Lipocalins are extracellular proteins that are secreted by cells; they were initially characterized as shuttle and transporter proteins. But it is now well established that members of the lipocalin family perform a variety of important functions by virtue of multiple molecular interactions underlying common molecular-recognition properties: binding to small hydrophobic molecules, binding to specific cell-surface receptors (e.g., plasma retinol-binding protein (RBP4) receptor STRA6, megalin, or lipocalin-2 receptor), and complexation with soluble macromolecules by non-covalent association or covalent link (Flower 1996).
The lipocalin family comprises a group of over 20 small (160–180 amino acid residues) glycoproteins. Most human lipocalins are clustered on the long arm of chromosome 9. Lipocalins exhibit only limited primary amino acid sequence similarity but share a highly conserved tertiary structure and several common sequence motifs. Their core structure...