Elucidation of African elephant beta casein phosphorylation state and casein micelle structure

[摘要] English: The exact structure of casein micelles still remains a debated subject. While most of the experimental work on cow caseins and casein micelles has provided a wealth of data, data of caseins and casein micelles of non-bovine origin provide a new insight into the structure of casein micelles. Microscopic examination of cow, sheep, horse, human and African elephant milk casein micelles show that the respective casein micelles are all spherical in shape but differ in size as well as surface appearances. Human casein micelles were the largest of the casein micelles whereas sheep casein micelles were the smallest. Apart from their smaller size, sheep micelles also had a smooth surface compared to a rough surface observed on the rest of the casein micelles. African elephant casein micelles were the second largest of the five casein micelles compared. It may be derived that, although casein micelle shape and size seem to be species specific, the differences observed may be a result of the differences in total casein content, the proportions of the individual casein types and the presence and or absence of some of the casein types.The elucidation of African elephant β-casein phosphorylation state by LC MS/MS, showed the presence of a single phosphorylation site at Ser9. In contrast, electrophoresis analysis showed that there are up to five phosphoforms of African elephant β-casein. The LC MS/MS also showed that the presence of a short length African elephant β-casein that is 200 amino acids long and that the gene sequences coded for by exons 4 and 5 have been truncated. Homology modeling of cow, sheep, horse, human and African elephant caseins showed that the secondary structure of α-caseins predominantly consist of α-helices, whereas the secondary structure of β- and κ-caseins is dominated by random coils. Alpha caseins give micelles a slightly compact structure whereas random coils result in a more open and larger size of micelles. These structural differences of caseins could possibly explain the varied size of casein micelles in milk. Comparative genomics of casein genes across mammalian species shows that several mammalian species are devoid of CNS1S1 and CSN1S2 genes. Considering the evolution of the casein gene locus organization, it appears that the CNS1S1 gene has been lost whereas the CSN1S2 gene has not been gained or developed in these species. In contrast, the CSN2 and CSN3 genes have been preserved and gained respectively, in most mammalian species. This suggests that these genes have a more important role in casein micelle formation and consequently the sequestration of large amounts of calcium and phosphate. Evidence from this study suggests that studying of non-cow caseins may shed more light on the casein micelle structure.

[发布机构] University of the Free State