Many biological processes involve globular transport proteins belonging to a family called lipocalins. The prominent feature in lipocalin structure is their specific tertiary conformation forming eight-stranded beta barrel with capacity to bind various ligands inside. The importance of lipocalins is evident from the list of vital substances (Hydrophobic ligands including vitamin A, steroids, bilins, lipids, pheromones etc.) that these proteins transport and from their high expression levels in various tissues. Among wide spectrum of lipocalins, Major Urinary Proteins (Mup) and Odorant Binding Proteins (Obp) are well known for their capacity to bind and carry odorants / pheromones and have been studied to detail in various mammalian models including mice, rats, and hamsters. However, many lipocalins (also including Mups) have previously been described with respect to their protective function in mammalian organism where they transport potentially harmful molecules to a degradation site (e. g. lysozomes) or straight out of the body. As most of lipocalins share similar tertiary structure, their potential role in both transport and excretion processes may be additive or complementary. In addition to a role of lipocalins in chemical communication this review presents lipocalins from the point of view of the "toxic waste hypothesis". This hypothesis assumes that members of lipocalins that are linked to a metabolic degradation of their ligands were an ideal source for natural selection during evolution due to an ability of potential receivers to detect lipocalin ligands levels as a signal by other individuals.
In humans, CD46 has been detected on the acrosomal membrane in sperm, in contrast to widespread surface expression on somatic cells where it plays a key role in the protection from complement attack. In rodents, CD46 is expressed solely on the acrosomal membrane of mature sperm and their immediate precursors, spermatids. A monoclonal antibody against the short consensus repeat (SCR1) ectodomain of CD46 blocks binding of human sperm to zona-free oocytes in vitro. However, CD46-knockout mice are fertile and have an accelerated spontaneous acrosome reaction. Wild-caught field mice (Apodemus) also exhibit a rapid acrosome reaction and CD46 is not expressed in Apodemus sperm. CD46 may, therefore, play a role in stabilization of the acrosomal membrane. Two other complement regulatory proteins, CD55 and CD59, are localized on the plasma membrane of mammalian sperm. In human sperm, CD55 and CD59 are expressed also on the inner acrosomal membrane. It remains to be clarified what is the role of CD46, CD55 and CD59 during fertilization and what are the advantages of not expressing CD46 in field mice sperm.