Sequence analysis of Camelus dromedarius milk caseins

Abstract

α(s1)-, α(s2)-, β- and κ-caseins from Somali camels (Camelus dromedarius) were purified by acid precipitation at pH 4.4, crudely separated into an α-CN and a β-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 α(s1)-CN 207, α(s2)-CN 178, β-CN 217, κ-CN 162. Percentage similarity to bovine proteins was α(s1)-CN A 39, α(s2)-CN 56, β-CN 64, κ-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) α(s1)-CN 220, α(S2)-CN 95, ̄-CN 650, ≃-CN 35. Degrees of phosphorylation and glycosylation were determined by laser ionization mass spectrometry and sequence pattern analysis. Molecular masses determined were (kDa) α(s1)-CN A, 24.755 and 24.668; α(s1)-CN B, 25.293; α(s2)-CN 21.993; ̄-CN, 24.900; κ-CN 22.294-22.987. The pH values of the most probable isoelectric points were: α(s1)-CN A 6P 4.41, α(s1)-CN B 6P 4.40, α(s2)-CN 9P 4.58, β-CN 4P 4.66, κ-CN 1P, with ten sialic acid residues bound, 4.10.