Sialyltransferases Catalyzing Mucin Biosynthesis

Studies on retention of SATs on the cell surface are of importance because of their possible role in intercellular adhesion (Roseman, 1970). The B-cell antigen CD-75 is reported to be a cell surface SAT (Stamenkovic et al., 1990) (see Chapter 2). The amino acid sequence of ST domains required for plasma membrane retention has not been established. Previous studies showed that a catalytically active ST6N (41-43 kDa) is secreted in serum, perhaps as a result of cleavage of Golgi membrane proteins by the action of a cathepsin D-like protease (Lammers and Jamieson, 1988). 

A soluble serum glycoproteinrSAT from inflamed rats has been reported by Mookerjea and his co-workers (Fraser et al., 1980, 1984). Although intestinal lymph SAT was increased by colchicine administration in rats, enterectomy did not prevent the rise of serum SAT, suggesting that the intestine is not a major source of the serum enzyme (Ratnam et al., 1981). However, the exact mechanism of the production of soluble serum glycoprotein SAT(s) from the membrane-bound Golgi or plasma membrane forms is unknown. 

The cDNA clones encoding ST6N have also been isolated from chick embryonic cDNA libraries using the conserved amino acid sequence information previously published in other systems (Kurosawa et al., 1994a). The embryonic chicken cDNA sequence revealed an open reading frame coding for 413 amino acids and contained 60% sequence identity with the rat liver enzyme. The ex- 

Recently, a cDNA encoding a GM3-specific a2-8 SAT (SAT-2 or GD3 synthase) has been isolated from an expression cDNA library of human melanoma cell line WM266-4 by enrichment of Namalwa KJM-1 cells highly expressing GD3 (Sasaki et al., 1994). Anti-GD3 antibody is used for fluorescence-activated 

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Four of the six different a2,3-sialyltransferases that catalyze the transfer of sialic acid residues in a2,3-linkage onto the terminal Galp residue are involved in mucin glycan chains biosynthesis (Table 3). 

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