β1,4-Galactosyltransferase and Lactose Biosynthesis: Recruitment of a Housekeeping Gene from the Nonmammalian Vertebrate Gene Pool for a Mammary Gland Specific Function

• Published in: Journal of mammary gland biology and neoplasia Vol. 3; no. 3; pp. 315 - 324
• Main Authors: Shaper, Nancy L., Charron, Martin, Lo, Neng-Wen, Shaper, Joel H.
• Format: Journal Article
• Language: English
• Published: Dordrecht Springer Nature B.V 01.07.1998
• Subjects: Biosynthesis; Gene pool; Glycoconjugates; Golgi apparatus; Lactose; Mammary gland; N-Acetyllactosamine
• ISSN: 1083-3021; 1573-7039
• DOI: 10.1023/A:1018719612087

β1,4-galactosyltransferase (β4GalT-I)is a constitutively expressed trans-Golgi enzyme, widelydistributed in vertebrates, which synthesizes theβ4-N-acetyllactosamine structure commonly found in glycoconjugates. In mammals β4GalT-Ihas been recruited for a second biosynthetic function,the production of lactose; this function takes placeexclusively in the lactating mammary gland. Inpreparation for lactose biosynthesis, β4GalT-I enzymelevels are increased significantly. We show that mammalshave evolved a two-step mechanism to achieve thisincrease. In step one there is a switch to the use of a second transcriptional start site,regulated by a stronger, mammary gland-restrictedpromoter. The transcript produced is distinguished fromits housekeeping counterpart by the absence of 180 nt of 5′-untranslated sequence. In step two,this truncated transcript is translated moreefficiently, relative to the major transcript expressedin all other somatic tissues.