STRUCTURAL-ANALYSIS OF THE GAP-RELATED DOMAIN FROM NEUROFIBROMIN AND ITS IMPLICATIONS

Neurofibromin is the product of the NF1 gene, whose alteration is resp onsible for the pathogenesis of neurofibromatosis type 1 (NF1), one of the most frequent genetic disorders in man. It acts as a GTPase activ ating protein (GAP) on Pas; based on homology to p120GAP, a segment sp anning 250-400 aa and termed GAP-related domain (NF1GRD; 25-40 kDa) ha s been shown to be responsible for GAP activity and represents the onl y functionally defined segment of neurofibromin, Missense mutations fo und in NF1 patients map to NF1GRD, underscoring its importance for pat hogenesis, X-ray crystallographic analysis of a proteolytically treate d catalytic fragment of NF1GRD comprising residues 1198-1530 (NF1-333) of human neurofibromin reveals NF1GRD as a helical protein that resem bles the corresponding fragment derived from p120GAP (GAP-334), A cent ral domain (NF1(c)) containing all residues conserved among RasGAPs is coupled to an extra domain (NF1(ex)), which despite very limited sequ ence homology is surprisingly similar to the corresponding part of GAP -334, Numerous point mutations found in NF1 patients or derived from g enetic screening protocols can be analysed on the basis of the three-d imensional structural model, which also allows identification of the s ite where structural changes in a differentially spliced isoform are t o be expected. Based on the structure of the complex between Pas and G AP-334 described earlier, a model of the NF1GRD-Ras complex is propose d which is used to discuss the strikingly different properties of the Ras-p120GAP and Ras-neurofibromin interactions.