USE OF AN ALTERNATE SPLICE DONOR SITE IN THE HUMAN GAP GENE IS RESPONSIBLE FOR SYNTHESIS OF THE P100 ISOFORM

GAP (GTPase-activating protein), a negative regulator of the receptor tyrosine kinase signal transduction pathway, exists as two isoforms: a ubiquitous, p120 form and a primate placenta-specific p100 form lacki ng the N-terminal hydrophobic domain. The cDNA species encoding p120 a nd p100 GAP are identical except that p100 GAP cDNA contains a 65-bp i nsert not present in p120 cDNA. The purpose of this study was to locat e the 65-bp insert in the genomic GAP sequence, thereby determining th e mechanism by which alternate splicing produces the two mRNA species. It was found that the 65-bp insert is located just 3' to the sequence encoding the hydrophobic domain, indicating that the p100 form of GAP results from utilization of an alternate splice donor site. In additi on, the sequence encoding the hydrophobic domain was found to be conta ined within a single large exon. The intron separating this exon from the exon encoding the 5'-portion of the SH2-N domain was determined to be at least 40 kb in length. Finally, it was found that the sequence encoding the SH2-N domain contains an intron 1006 bp long, and the seq uence of this intron has been deduced. It is anticipated that the data presented in this paper will provide the basis for elucidating RNA pr ocessing mechanisms responsible for preferential expression of p100 GA P in the human placenta.