Lipocalins are proteins that bind to and transport small hydrophobic molecules such as lipids and steroids, and have been associated with biological processes such as milk production, pheromone transport and immune responses.

Lipocalins are a family of proteins that bind to and transport small hydrophobic molecules such as lipids and steroids, and have been associated with biological processes such as milk production, pheromone transport and immune responses. Lipocalins share a common structure of eight folded arms enclosing an internal ligand-binding site, and are found in vertebrates, invertebrates, plants and certain bacteria (gram negatives).

Milk secretions

Lipocalin proteins are a key component of milk secreted to feed young in both placentals and marsupial mammals, and it appears that these proteins have been convergently recruited in ‘milk’ secretions of some viviparous or ovoviviparous insects. For example, the viviparous cockroach Diploptera punctata keeps its young in brood sac, the cells of which produce a variety of lipocalins, carrying essential nutrients to the embryos.

Pheromone transport

The ability of lipocalins to carry hydrophobic compounds makes them suitable to transport pheromones (communication chemicals which are themselves convergent). In this context it is employed by some male cockroaches to stimulate the female, who consumes the integumentary product, and in conjunction with stereotyped behavioural displays, leads to mating. In the cockroach Leucophaea maderae, aphrodisiac lipocalins are secreted from a tergal gland that is made of an infolded membrane reminiscent of the Diploptera brood sac, suggestive of convergent morphology and physiology. So too in some mammals lipocalins are employed to carry pheromones, such as the appropriately named aphrodisin secreted by the female hamster, and a range of lipocalins found in the nasal and urinary secretions of rats and mice.

Allergenic activity

In mammals lipocalins can trigger immune responses, in particular inflammation and other allergen reactions. In a bizarre parallel, lipocalins of the cockroach Blatella germanica also show allergenic activity, indicating a possible convergence between these proteins in insects and vertebrates.