THE UNUSUAL AMINO-ACID TRIPLET ASN-ILE-CYS IS A GLYCOSYLATION CONSENSUS SITE IN HUMAN ALPHA-LACTALBUMIN

Human alpha-lactalbumin has not been described as a glycoprotein, desp ite the fact that several alpha-lactalbumins of both ruminant and nonr uminant species are known to be glycosylated. In all these species the glycosylation site is the (45)Asn in the usual triplet (45)Asn-Gly/Gl n-(47)Ser. We have found that human alpha-lactalbumin is glycosylated and the glycosylation site has been determined by protein sequencing a nd mass spectrometry. We report an unusual glycosylation site at (71)A sn in the triplet (71)Asn-Ile-(73)Cys, which is conserved in all known alpha-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 globul e state of alpha-lactalbumin.