Abstract
Three sialyltransferases which attach terminal sialic acids to glycoprotein sugar chains are shown to exhibit striking differential expression in seven tissues of the rat. Using cDNA probes for the Gal beta 1,4GlcNAc alpha-2,6-sialytransferase which forms a NeuAc alpha 2-6Gal beta 1-4GlcNAc sequence on N-linked sugar chains, three different sized mRNAs are detected, two of which (4.7 and 4.3 kilobases (kb] have high homology along the full length, and a third (3.6 kb, in kidney) which is missing the 5' region corresponding to 45% of the NH2-terminal coding sequence. The 4.7- and 4.3-kb mRNAs exhibit differential expression of over 50-fold with the highest levels in liver and lowest in brain and heart. Assays for enzyme activity in tissue homogenates show high correspondence to the levels of mRNA. Evidence of tissue-specific expression was also obtained for two other sialyltransferases which form the NeuAc alpha 2-3Gal beta 1-4/3GlcNAc and NeuAc alpha 2-3Gal beta 1-3GalNAc sequences on N-linked and O-linked sugar chains, respectively. Comparison of the ratios of the three enzymes in several tissues suggests that they are expressed independently. The results are discussed for their relevance to cellular control of terminal glycosylation sequences on glycoproteins and glycolipids.
Highlights
From the Department of Biological Chemistry, Because the carbohydrate structuresof these glycoconjugates and Molecular Biology Institute, UCLA School of are largelydetermined by the specificitiesof the glycosyltrans
What is the consequence of the differential expression of terminal glycosyltransferases? Most terminal glycosyltransferases compete for common acceptor substrates [15]
The type of terminal glycosyltransferases expressed by a cell should influence the types of terminal sequences found on the carbohydrate groups it produces
Summary
0 1989 by The American Society for Biochemistry and Molecular Biolo Inc. Printed in%S.A. The. sialytransferase, and theGalp1,BGalNAca-2,3-sialytransferase were Tissue and cell type-specific expression of terminal glyco- purified as reported previously [28, 29]. Antibodies directed to specific carbohydrate structuresdetect cell surface antigens on some normal tissues but not others anodn tumor tissues where the same carbohydrate antigens are not present on surrounding normal tissue [1,2] Some of these carbohydrate structures pared as described previously [28, 30]. Northern Analysis-Total mRNA was prepared from freshly dissected rat tissues as previously described [31].Gel electrophoresis of total RNA (15 pg/lane) was done in 1%agarose gels containing formaldehyde, and Northern hybridizations were performed as reported earlier [23]. Homogenates were either assayed immediately or aliquoted and stored frozen a t -80 "C and thawed immediately
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