The EXT1/EXT2 tumor suppressors: catalytic activities and role in heparan sulfate biosynthesis

The D-glucuronyltransferase and N-acetyl-D-glucosaminyl-transferase reactio ns in heparan sulfate biosynthesis have been associated with two genes, EXT 1 and EXT2, which are also implicated in the inherited bone disorder, multi ple exostoses. Since the cell systems used to express recombinant EXT prote ins synthesize endogenous heparan sulfate, and the EXT proteins tend to ass ociate, it has not been possible to define the functional roles of the indi vidual protein species. We therefore expressed EXT1 and EXT2 in yeast, whic h does not synthesize heparan sulfate. The recombinant EXT1 and EXT2 were b oth found to catalyze both glycosyltransferase reactions in vitro. Coexpres sion of the two proteins, but not mixing of separately expressed recombinan t EXT1 and EXT2, yields hetero-oligomeric complexes in yeast and mammalian cells, with augmented glycosyltransferase activities. This stimulation does not depend on the membrane-bound state of the proteins.