Selective impairment of the synthesis of basic fibroblast growth factor binding domains of heparan sulphate in a COS cell mutant defective in N-sulphotransferase

N-Sulphation is a key step in the overall sulphation of heparansulphate. We have isolated a COS cell-derived mutant, CM-15,that is impaired in its ability to bind to basic fibroblastgrowth factor (bFGF) and has a 2- to 3-fold reduction in N-sulphotransferaseactivity Ishihara et al., (1992a) Anal. Biochem., 206, 400–407].We now provide structural evidence that CM-15 is selectivelyimpaired in the synthesis of highly sulphated regions or ‘blocks’that display high-affinity binding to bFGF; these are completelyN-sulphated blocks of decasaccharide or greater length thatare enriched in O-sulphate groups. The synthesis of sulphatedblocks that did not show high affinity to the growth factorwas relatively unimpaired in the mutant cells; this includedfully N-sulphated octamer (or smaller) blocks and, unexpectedly,decasaccharide or larger blocks that were poorly O-sulphated.In the latter fraction, the failure to form high-affinity bindingregions was the result of a failure to stimulate O-sulphationrather than N-sulphation in CM-15 cells. In agreement with otherstudies, disaccharide analysis of the wild-type-derived sulphatedblocks suggested that 2-O-sulphation of iduronate residues inthe polymer was a necessary element to produce a high-affinitybinding sequence once N-sulphation was completed in the decasaccharideor larger fraction. These results suggest that a selective reductionin both N- and O-sulphation in the larger blocks produced byCM-15 cells is a consequence of the reduction of N-sulphotransferaseactivity. These data provide a potential mechanism for regulatingthe synthesis of high-affinity bFGF binding domains in the heparansulphate of mammalian cells. basic fibroblast growth factor COS cell mutant heparan sulfate N-sulphotransferase