Contributed by: Kazuhiko Kawasaki
In most mammals, caseins are the most abundant proteins in milk. Caseins associate with calcium and stabilize it at a high concentration, which helps infants to develop bone and teeth. Despite the critical role of caseins in mammalian evolution, their origin has been unknown or highly speculative. Association of caseins with calcium is mediated by negatively charged amino acids, and no rigid 3-dimensional structure is required for this function. Mainly for this reason, caseins evolve rapidly, and hence the amino acid or nucleotide sequence similarity is not very useful for exploring their evolution. For this reason, we studied similarities in the exon-intron structure of the genes, which is more conserved during evolution (1). The results suggest that two of the three casein genes have evolved by duplication of the SCPPPQ1 gene, whereas the third casein gene originated from the FDCSP gene. In addition, the SCPPPQ1 and FDCSP genes share a common ancestral gene called ODAM. Interestingly, all these three precursor genes are expressed in dental tissues and probably encode calcium-binding proteins. We therefore argue that all casein genes arose from tooth genes and that the calcium-binding ability of caseins was inherited from their ancestor.
SCPPPQ1: secretory calcium-binding phosphoprotein proline and glutamine rich 1
FDCSP: follicular dendritic cell secreted peptide
Caseins are among cardinal proteins that evolved in the lineage leading to mammals. In milk, caseins and calcium phosphate (CaP) form a huge complex, called casein micelle. By forming the micelle, milk maintains high CaP concentrations, which help altricial mammalian neonates to grow bone and teeth. Two types of caseins are known. Ca-sensitive caseins (as- and b-caseins) bind Ca but precipitate at high Ca concentrations, whereas Ca-insensitive casein (k-casein) does not usually interact with Ca but instead stabilizes the micelle. Thus, it is thought that these two types of caseins are both necessary for stable micelle formation. Both types of caseins show high substitution rates, which make it difficult to elucidate the evolution of caseins. Yet, recent studies have revealed that all casein genes belong to the secretory calcium-binding phosphoprotein (SCPP) gene family that arose by gene duplication. In the present study, we investigated exon-intron structures and phylogenetic distributions of casein and other SCPP genes, particularly the odontogenic ameloblast associated (ODAM) gene, the SCPP-Pro-Gln-rich 1 (SCPPPQ1) gene, and the follicular dendritic cell secreted peptide (FDCSP) gene. The results suggest that contemporary Ca-sensitive casein genes arose from a putative common ancestor, which we refer to as CSN1/2. The six putative exons comprising CSN1/2 are all found in SCPPPQ1, although ODAM also shares four of these exons. By contrast, the five exons of the Ca-insensitive casein gene are all reminiscent of FDCSP. The phylogenetic distribution of these genes suggests that both SCPPPQ1 and FDCSP arose from ODAM. We thus argue that all casein genes evolved from ODAM via two different pathways; Ca-sensitive casein genes likely originated directly from SCPPPQ1, whereas the Ca-insensitive casein genes directly differentiated from FDCSP. Further, expression of ODAM, SCPPPQ1, and FDCSP was detected in dental tissues, supporting the idea that both types of caseins evolved as Ca-binding proteins. Based on these findings, we propose two alternative hypotheses for micelle formation in primitive milk. The conserved biochemical characteristics in caseins and their immediate ancestors also suggest that many slight genetic modifications have created modern caseins, proteins vital to the sustained success of mammals.
1. Kawasaki, K., Lafont, A.G., and Sire, J.Y. 2011. The evolution of milk casein genes from tooth genes before the origin of mammals. Mol. Biol. Evol. 28(7):2053-2061.