Alternative Splicing of Lactophorin mRNA from Lactating Mammary Gland of the Camel (Camelus dromedarius)

The objective of this study was to determine the corrected structure of lactophorin, a major whey protein in camel milk. The protein had 60.4% amino acid sequence identity to a proteose peptone component 3 protein from bovine whey and 30.3% identity to the glycosylation-dependent cell adhesion molecule 1 in mice. The N-terminal heterogeneity of the protein was a result of alternative mRNA splicing. About 75% of the protein was expressed as a long variant A with 137 amino acid residues and a molecular mass of 15.7 kDa; about 25% was as a short variant B with 122 amino acid residues and a molecular mass of 13.8 kDa. Both proteins are probably threefold phosphorylated. In contrast to the related proteins, no glycosylation was found in camel lactophorin. Because of this difference, specific interaction with carbohydrate binding proteins, as reported for the murine protein, can be excluded, and a function of the protein other than cell recognition or rotaviral inhibition is proposed. The concentration of lactophorin in camel milk was found to be about three times higher than the concentration of the bovine homologue in bovine milk. Pronounced similarities existed between the primary and secondary structures of bovine and camel proteins. We speculated that camel lactophorin has a similar function to that of bovine protein in milk, which is supposed to be the prevention of fat globule aggregation and the inhibition of spontaneous lipolysis by lipoprotein lipase.