The following sequence of terminal sugar residues: Gal-alpha1,3-Gal-beta1,4-GlcNAc- (alpha-Gal epitope), is normally not found in humans, but has been found in mutant human cell lines (Sharon 1993, and references therein). Antibodies against this motif are part of the human complement system and are thought to serve as a protection against retroviruses expressing the alpha-Gal epitope on their surfaces.
Several other uncommon structures with varying types of linkages and different sugar than mentioned so far, incorporated into the oligosaccharide sidechain have been found, e.g. arabinose attached the core Man is commonly found in tomatoes. For all these types of sidechains special glycosyltransferase probably exist in the Golgi apparatus, but the biosynthetic pathways have not been elucidated so far.
More than 150 mammalian glycosyltransferases have been characterized with a predicted general composition typical for a type II transmembrane topology: i.e. a short cytoplasmatic tail (NH2-terminal) followed by a transmembrane part which is separated by a short 'stem' region from the large cytoplasmic catalytic domain (COOH-terminal). (Paulson 1989, Lowe 1994)
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Written by: Christian Frosch
frosch@mzdmza.zdv.uni-mainz.de
http://www.uni-mainz.de/~frosc000
Institute of Toxicology
University of Mainz
Last update: 11.07.1995