Sequence analysis of Camelus dromedarius milk caseins.

Alpha s1-, alpha s2-, beta- and kappa-caseins from Somali camels (Camelus dromedarius) were purified by acid precipitation at pH 4.4, crudely separated into an alpha-CN and a beta-CN fraction and further purified by reversed-phase HPLC. Fragments of tryptic digests were sequenced. Amino acid patterns obtained were used to screen a cDNA library constructed from mRNA from lactating udder tissue. Full length clones corresponding to the four caseins were sequenced. The numbers of residues in the sequences deduced were alpha s1-CN 207, alpha s2-CN 178, beta-CN 217, kappa-CN 162. Percentage similarity to bovine proteins was alpha s1-CN A 39, alpha s2-CN 56, beta-CN 64, kappa-CN 56. Acid-precipitated casein of pooled milk was separated by reversed-phase HPLC and monitored at 220 nm, and its composition, estimated from peak integration, was (g/kg total casein) alpha s1-CN 220, alpha s2-CN 95, beta-CN 650, kappa-CN 35. Degrees of phosphorylation and glycosylation were determined by laser ionization mass spectrometry and sequence pattern analysis. Molecular masses determined were (kDa) alpha s1-CN A, 24.755 and 24.668; alpha s1-CN B, 25.293; alpha s2-CN 21.993; beta-CN, 24.900; kappa-CN 22.294-22.987. The pH values of the most probable isoelectric points were: alpha s1-CN A 6P 4.41, alpha s1-CN B 6P 4.40, alpha s2-CN 9P 4.58, beta-CN 4P 4.66, kappa-CN 1P, with ten sialic acid residues bound, 4.10.