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

Abstract

N-Sulphation is a key step in the overall sulphation of heparan sulphate. We have isolated a COS cell-derived mutant, CM-15, that is impaired in its ability to bind to basic fibroblast growth factor (bFGF) and has a 2- to 3-fold reduction in N-sulphotransferase activity [Ishihara et al., (1992a) Anal. Biochem., 206, 400–407]. We now provide structural evidence that CM-15 is selectively impaired in the synthesis of highly sulphated regions or ‘blocks’ that display high-affinity binding to bFGF; these are completely N-sulphated blocks of decasaccharide or greater length that are enriched in O-sulphate groups. The synthesis of sulphated blocks that did not show high affinity to the growth factor was relatively unimpaired in the mutant cells; this included fully 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 binding regions was the result of a failure to stimulate O-sulphation rather than N-sulphation in CM-15 cells. In agreement with other studies, disaccharide analysis of the wild-type-derived sulphated blocks suggested that 2-O-sulphation of iduronate residues in the polymer was a necessary element to produce a high-affinity binding sequence once N-sulphation was completed in the decasaccharide or larger fraction. These results suggest that a selective reduction in both N- and O-sulphation in the larger blocks produced by CM-15 cells is a consequence of the reduction of N-sulphotransferase activity. These data provide a potential mechanism for regulating the synthesis of high-affinity bFGF binding domains in the heparan sulphate of mammalian cells.