Bovine seminal RNase (BS-RNase) is a member of the ribonuclease superfamily produced by the bovine seminal vesicles. This enzyme can not be differentiated from its members distinctly since there are more features that this enzyme shares with its family members than features that it possess alone. The research on the question of how new functions arrive in proteins in evolution led the scientists to find an uncommon consequence for a usual biological event called gene conversion in the case of the ribonuclease (RNase) protein family. The most well-known member of this family, RNase A (also called pancreatic RNase), is expressed in the pancreas of oxen. It serves to digest RNA in intestine, and evolved from bacteria fermenting in the stomach of the first ox. The homologous RNase, called seminal RNase, differs from RNase A by 23 amino acids and is expressed in seminal plasma in the concentration of 1-1.5 mg/ml, which constitutes more than 3% of the fluid protein content. Bovine seminal ribonuclease (BS-RNase) is a homologue of RNase A with specific antitumor activity.
Functional properties of bovine seminal RNase
The physiological role of this enzyme is not yet found and thus it is still a mystery why the seminal fluid in bovine has such a higher concentration of this enzyme. In the evolutionary process, it has acquired new behaviors such as being a dimer with composite active sites binding firmly to anionic glycolipids, including bovine spermatozoa seminolipid, a fusogenic sulfated galactolipid possessing immunosuppressive and cytostatic activities whereas the ancestral RNase does not possess these behaviors. The homolog of RNase A, bovine seminal ribonuclease (BS-RNase), has a specific antitumor activity. In the immunoregulation of both male and female genital systems, the seminal plasma plays a prominent role in immunosuppression. The direct or indirect interference of the seminal plasma with the function of many types of immunocompetent cells including T cells, B cells, NK cells and macrophages has been shown. These effects of immunosuppression are not species-specific and are found to be in physiological concentrations that are normally seen in the urogenital tract of females. RNase secretion has not been detected in the seminal fluid of any other mammal.
Origin of seminal RNase gene
The recruitment of established proteins after the gene duplication leads to play some new biomolecular functions. Among different models that exist, one model suggests that after the gene duplication, among the two copies of genes, one will be subjected to continuous evolution under ancestral dictated functional constraints whereas the duplicate meanwhile will not be restricted by a functional role and feels free to find protein “structure space”. In the end, it may come with encoded new behaviors that which are required for a new physiological function and thereby discourse the selective advantage. In any case, we may consider it as an ambiguous model since most duplicates have to become pseudogenes, which are considered as an inexpressible genetic information (referred to as “junk DNA”) in just a few million years. Because selective pressure can do nothing much with duplicated genes, they are prone to deleterious mutations that present their incapability to encode a protein useful for any function. This restricts to use a functionally unconstrained gene duplicate as a tool for investigating protein structure space of new behaviors that might discourse selectable physiological function. Then how would new functions arise in proteins? One of the possibilities is the resurrection of the pseudogenes due to some biological events like gene conversion. One such an example is the resurrection of the bovine seminal RNase gene.