The unusual amino acid triplet Asn-IIe-Cys is a glycosylation consensus site in human α-lactalbumin

Human α-lactalbumin has not been described as a glycoprotein, despite the fact that several α-lactalbumins of both ruminant and nonruminant species are known to be glycosylated. In all these species the glycosylation site is the ⁴⁵Asn in the usual triplet ⁴⁵Asn-Gly/Gln-⁴⁷Ser. We have found that human α-lactalbumin is glycosylated and the glycosylation site has been determined by protein sequencing and mass spectrometry. We report an unusual glycosylation site at ⁷¹Asn in the triplet ⁷¹Asn-Ile-⁷³Cys, which is conserved in all known α-lactalbumins except red-necked wallaby. That a relatively small proportion of the protein is glycosylated (about 1%) may reflect the importance of this region of the protein sequence to the molten globule state of α-lactalbumin.