Lipocalin-1 (LCN1, also known as tear lipocalin, von ebner's gland protein, or tear specific prealbumin), the main lipid carrier in human tears, is critical to functions involving lipids in protection of the ocular surface. Its large ligand pocket accommodates a range of ligands including alkyl alcohols, glycolipids, phospholipids, cholesterol, steroids, and siderophores. Lipocalin-3 (LCN3, also known as vomeronasal secretory protein 1) and lipocalin-4 (LCN4, also known as vomeronasal secretory protein 2) are involved in transport of lipophilic molecules, and are possibly pheromone-carriers. Lipocalin-13 (LCN13, also known as odorant binding protein 2A) may bind and transport small hydrophobic volatile molecules with a higher affinity for aldehydes and large fatty acids. Another member of this family is late lactation protein B (LLPB), a milk protein produced during the late phase of lactation, which may be involved in transporting a small ligand released during the hydrolysis of milk fat. This group belongs to the lipocalin/cytosolic fatty-acid binding protein family which have a large beta-barrel ligand-binding cavity. Lipocalins are mainly low molecular weight extracellular proteins that bind principally small hydrophobic ligands, and form covalent or non-covalent complexes with soluble macromolecules, as well as membrane bound-receptors. They participate in processes such as ligand transport, modulation of cell growth and metabolism, regulation of immune response, smell reception, tissue development and animal behavior. Cytosolic fatty-acid binding proteins, also bind hydrophobic ligands in a non-covalent, reversible manner, and have been implicated in intracellular uptake, transport and storage of hydrophobic ligands, regulation of lipid metabolism and sequestration of excess toxic fatty acids, as well as in signaling, gene expression, inflammation, cell growth and proliferation, and cancer development.
Jiyao W et al.(2023). "The conserved domain database in 2023.",