A Conserved Alternative Splice inthevon Recklinghausen Neurofibromatosis (NF1)GeneProduces TwoNeurofibromin Isoforms, BothofWhichHaveGTPase-Activating Protein Activity

protein (GAP),yeastIralpandIra2p(inhibitory regulators oftheRAS-cyclic AMP pathway), andneurofibromin, theprotein encodedbytheNFl gene.Yeastexpression experiments have confirmed that a 381-amino-acid segment ofneurofibromin, dubbedtheGAP-related domain(GRD), can function asa GAP.UsingtheRNA polymerase chainreaction withprimers flanking theNFI-GRD, we have identified evidence foralternative splicing inthis region oftheNFlgene.Inaddition tothealready published sequence(type I), an alternative RNA carrying a 63-nucleotide insertion (type II)ispresent inalltissues examined, although therelative amountsoftypesIandII vary.Theinsertion isconserved acrossspecies but isnotpresent inGAP,IRA], orIRA2.GenBanksearches havefailed toidentify significant similarity between theinserted sequenceandknownDNA orprotein sequences,although thebasic aminoacidcomposition ofthe insertion shares features withnuclear targeting sequences.Expression studies inyeasts showthatdespite the partial disruption oftheneurofibromin-IR4-GAP homology bythis insertion, bothformsoftheNFI-GRDcan complement loss ofIRAfunction. Invivo assaysdesigned tocomparetheGAPactivity ofthetwoalternatively spliced formsoftheNFI-GRDshowthat bothcanincrease theconversion ofGTP-bound rastoitsGDP-bound form,although theinsertion ofthe21aminoacids weakensthiseffect. Thestrong conservation ofthis alternative splicing suggests thatbothtypeIandII isoforms mediate important biological functions of neurofibromin.