Genetic variation in the β-lactoglobulin of Chinese yak (Bos grunniens)

Abstract

Yak (Bos grunniens) is distributed in the area of Central Asian highlands, it thrives in conditions of extreme harshness with severely cold winters, short growing seasons for herbage and no absolutely frost-free periods (Wiener et al. 2003). The total population of yak is estimated to be 14 million, about 90% of the domestic yaks are found on the plateau of western China, with the largest population in the Sichuan province (Wiener et al. 2003; Zhang et al. 2008). In contrast to the milk of dairy cattle and goat, yak’s milk contains a higher percentage of protein, organic calcium, and conjugated linoleic acid (Wiener et al. 2003; Li et al. 2010). β-lactoglobulin (β-LG) is an important protein that constitutes approximately up to 65% of the total whey protein of bovine milk. β-LG is coded by LGB gene, located on chromosome 11 of the bovine genome (Hayes and Petit 1993). It is a single-stranded protein of 18 kDa comprising of 162 amino acid residues, and is stabilized by five disulphide bonds (Apenten et al. 2002). β-LG is a member of the lipocalin protein superfamily with the function of transporter of hydrophobic molecules (Flower et al. 2000). The ability of β-LG to bind hydrophobic and amphiphilic molecules has been confirmed, ranging from hexane to palmitic acid to retinol to vitamin D (Perez and Calvo 1995; Narayan and Berliner 1997; Kontopidis et al. 2004). Other biological functions of β-LG have been reported, including enzyme regulation, the neonatal acquisition of passive immunity, source of bioactive peptides and antimicrobial activity against mastitis-causing bacteria (Perez et al. 1992; Kontopidis et al. 2004; Pellegrini et al. 2004; Hernandez-Ledesma et al. 2008; Chaneton et al. 2011). Milk proteins show genetic polymorphism due to nucleotide sequence substitution or deletion, various degrees