The Evolution of Milk Casein Genes from Tooth Genes before the Origin of Mammals

Abstract

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 (α(s)- and β-caseins) bind Ca but precipitate at high Ca concentrations, whereas Ca-insensitive casein (κ-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.

The evolution of milk secretion and its ancient origins

“…The proposal that mammary secretion has an ancient origin and long evolutionary history (Oftedal, 2002a and2002b) is increasingly accepted, especially in the face of supportive molecular evidence (Kawasaki and Weiss, 2003;Vorbach et al, 2006;Brawand et al, 2008;McClellan et al, 2008;Capuco and Akers, 2009;Lemay et al, 2009;Lefevre et al, 2010;Kawasaki et al, 2011). It is now possible to formulate a more detailed scenario by which the secreted fluid came to resemble in form and function what we now know as milk.…”

“…As unfolded proteins, all SCPPs are low in cysteine and therefore cystine disulfide bridges, and a subclass of the proteins (P/Q-rich SCPPs), including the caseins, are particularly rich in proline and glutamine (Kawasaki and Weiss, 2003;Kawasaki et al, 2011). On the basis of the relative locations and structures of exons of these P/Q-rich SCPPs, as well as their phylogenetic distribution, Kawasaki et al (2011) proposed that the a-and b-caseins derived via gene duplication and exon changes from an ancestral gene (CSN1/2) that derives from another SCPP gene, either ODAM or SCPPPQ1 (which itself is derived from ODAM ), whereas k-casein derives from the SCPP gene FDCSP (which is also derived from ODAM ).…”

“…Many P/Q-rich SCPPs, including the ODAM and SCPPPQ1 derived proteins, are expressed in mammalian ameloblasts and are involved in mineralization of tooth enamel; follicular dendritic cell secreted peptide (from FDCSP ) is found in soft connective tissue (periodontal ligament), where it is thought to prevent spontaneous precipitation of calcium phosphate; it is also expressed in the mammary gland (Kawasaki, 2009;Kawasaki et al, 2011). Kawasaki et al (2011) suggest that the initial function of an ancestral SCPP (probably a k-casein precursor) in protolacteal secretion may have been to regulate calcium delivery to the surface of an egg and to prevent precipitation of calcium phosphate on the parchment-like eggshell.…”

“…Kawasaki et al (2011) suggest that the initial function of an ancestral SCPP (probably a k-casein precursor) in protolacteal secretion may have been to regulate calcium delivery to the surface of an egg and to prevent precipitation of calcium phosphate on the parchment-like eggshell. Kawasaki et al (2011) hypothesize that this may have occurred before the divergence of sauropsids and synapsids, although an ancestral CSN1/2 has yet to be found in a sauropsid genome.…”

Invited review: Caseins and the casein micelle: Their biological functions, structures, and behavior in foods

“…Caseins evolved from members of a group of secreted calcium (phosphate)-binding phosphoproteins (SCPP; Supplementary File, sections S1 and S2, available online at http://dx.doi.org/10.3168/jds.2013-6831; Kawasaki and Weiss, 2003;Rijnkels et al, 2003;Kawasaki et al, 2004Kawasaki et al, , 2011Lemay et al, 2009). In eutherian milks, at least 3 and normally 4 gene products are found; namely, α S1 -, α S2 -, β-, and κ-CN, but in some species 2 quite different α S2 -CN-like genes are active, raising the total number of gene products to as many as 5.…”