| Summary
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ST6Gal I is an enzyme belonging to the sialyltransferase family [Tsuji et al. 1996]
and forms Sia-a-2,6-Gal linkage on the Gal-b-1,4-GlcNAc-sequence present on N-linked glycoproteins. So far, 16 enzymes have been cloned [Tsuji, S 1996]
; [Lee et al. 1999]
; [Okajima et al. 1999]
, each of which exhibits unique specificity for its acceptor substrates and forms one of four sialic acid linkages, namely, Neu5Aca2,6Gal, Neu5Aca2,3Gal, Neu5Aca2,6GalNAc, or Neu5Aca2,8Neu5Ac.
ST6Gal I forms Neu5Aca2,6Gal linkage. This and other sialyltransferases are localized in the Golgi apparatus [Taatjes et al. 1987]
and are type II membrane proteins with a short cytoplasmic domain, an N-terminal signal anchor, a "stem" region, and a large luminal domain that confers the catalytic activity. Another structural feature of ST6Gal I that is common among other sialyltransferases despite little homology is the presence of two conserved protein domains, termed, L-(Long) and S-(Short) sialylmotif [Datta et al. 1997]
. Analysis by site-directed mutagenesis showed that these two motifs are linked by an essential disulfide linkage and important for substrate binding [Datta et al. 2001]
. In addition, a very small motif of unknown function is found at the C-terminal [Geremia et al. 1997]
. No structural information, however, is available from X-ray crystallography or NMR studies.
The product of this ST6Gal I, Neu5Aca2,6Galb1,4GlcNAc is shown to be the ligand of CD22
, a receptor present on B cells [Powell et al. 1994]
. In vivo functional study using ST6Gal I deficient mice indicated that this sialoside is essential in promoting B lymphocyte activation and immune function [Hennet et al. 1998]
.
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