Milk protein genetic variants and isoforms identified in bovine milk representing extremes in coagulation properties

Abstract

A gel-based proteomic approach consisting of 2-dimensional gel electrophoresis coupled with mass spectrometry was applied for detailed protein characterization of a subset of individual milk samples with extreme rennet coagulation properties. A milk subset with either good or poor coagulation abilities was selected from 892 Danish Holstein-Friesian and Jersey cows. Screening of genetic variants of the major milk proteins resulted in the identification of common genetic variants of β-casein (CN; A(1), A(2), B), κ-CN (A, B), and β-lactoglobulin (LG; A, B), as well as a low frequency variant, κ-CN variant E, and variants not previously reported in Danish breeds (i.e., β-CN variant I and β-LG variant C). Clear differences in the frequencies of the identified genetic variants were evident between breeds and, to some extent, between coagulation groups within breeds, indicating that an underlying genetic variation of the major milk proteins affects the overall milk coagulation ability. In milk with good coagulation ability, a high prevalence of the B variants of all 3 analyzed proteins were identified, whereas poorly coagulating milk was associated with the β-CN variant A(2), κ-CN variant A or E, and β-LG variant A or C. The β-CN variant I was identified in milk with both good and poor coagulation ability, a variant that has not usually been discriminated from β-CN variant A(2) in other studied cow populations. Additionally, a detailed characterization of κ-CN isoforms was conducted. Six κ-CN isoforms varying in phosphorylation and glycosylation levels from each of the genetic variants of κ-CN were separated and identified, along with an unmodified κ-CN form at low abundance. Relative quantification showed that around 95% of total κ-CN was phosphorylated with 1 or 2 phosphates attached, whereas approximately 35% of the identified κ-CN was glycosylated with 1 to 3 tetrasaccharides. Comparing isoforms from individual samples, we found a very consistent κ-CN isoform pattern, with only minor differences in relation to breed, κ-CN genetic variant, and milk coagulation ability.

Genetic variation and posttranslational modification of bovine κ-casein: Effects on caseino-macropeptide release during renneting

“…Variations in glycosylation degree of κ-CN and of κ-CN isoforms have been addressed in several studies (Dalgleish, 1986;Robitaille et al, 1991;Coolbear et al, 1996;Jensen et al, 2012a). In a recent larger study, no significant difference was observed between genetic variants, although κ-CN BB was found to have a slightly higher glycosylation degree compared with κ-CN AB and AA (Bonfatti et al, 2014).…”

Association between the bovine milk metabolome and rennet-induced coagulation properties of milk

“…Some of the most important factors influencing coagulation properties are the total fat and protein contents, in addition to variability in protein composition and specific genetic variants of the caseins (Wedholm et al, 2006;Hallén et al, 2007;Jensen et al, 2012). Moreover, SCC, acidity (Ikonen et al, 2004), and variability in minor milk compounds, such as minerals and citrate (Udabage et al, 2001;Tsioulpas et al, 2007), have also been associated with differential milk coagulation properties.…”

Heritability and repeatability of milk coagulation properties predicted by mid-infrared spectroscopy during routine data recording, and their relationships with milk yield and quality traits

“…Improvement of these traits can be achieved through selection for correlated traits such as milk protein content and composition (Bonfatti et al, 2011;Jensen et al, 2012), milk acidity (Ló pez et al, 1998; -E-mail: martino.cassandro@unipd.it Ná jera et al, 2003) and somatic cell count (SCC; Politis and Ng-Kwai-Hang, 1988;Klei et al, 1998). Moreover, genetic variants at the protein loci have been demonstrated to exert an effect on MCP (Kü barsepp et al, 2005;Comin et al, 2008).…”