The joys of HexNAc. The synthesis and function of N-and O-glycan branches

This review covers discoveries made over the past 30-35 years that were imp ortant to our understanding of the synthetic pathway required for initiatio n of the antennae or branches on complex N-glycans and O-glycans. The revie w deals primarily with the author's contributions but the relevant work of other laboratories is also discussed. The focus of the review is almost ent irely on the glycosyltransferases involved in the process. The following to pics are discussed. (1) The localization of the synthesis of complex N-glyc an antennae to the Golgi apparatus. (2) The "evolutionary boundary" at the stage in N-glycan processing where there is a change from oligomannose to c omplex N-glycans; this switch correlates with the appearance of multicellul ar organisms. (3) The discovery of the three enzymes which play a key role in this switch, N-acetylglucosaminyltransferases I and II and mannosidase I I. (4) The "yellow brick road" which leads from oligomannose to highly bran ched complex N-glycans with emphasis on the enzymes involved in the process and the factors which control the routes of synthesis. (5) A short discuss ion of the characteristics of the enzymes involved and of the genes that en code them. (6) The role of complex N-glycans in mammalian and Caenorhabditi s elegans development. (7) The crystal structure of N-acetylglucosaminyltra nsferase I. (8) The discovery of the enzymes which synthesize O-glycan core s 1, 2, 3 and 4 and their elongation.