Determinants of species- and placenta-specific expression of p100 GAP

This study was based on the hypothesis that both primary sequence and methylation status of the GTPase activating protein (GAP) gene limits expression of p100 GAP to primate placenta. Due to alternate splicing, a 65-bp insert appears between the first and second coding exons of p100 GAP mRNA, and translation of p100 GAP initiates within this insert. Examination of the sequence surrounding the 65-bp insert revealed that the monkey GAP gene contained both the 3' splice donor site and the internal start codon, whereas the mouse GAP gene contained neither. To address p100 GAP tissue specificity, the methylation status of the GAP gene was examined. Site-specific demethylation was found to correlate with synthesis of p100 GAP, suggesting that methylation regulates the expression of different GAP isoforms. The results of this study provide a mechanistic basis for the observation that p100 GAP is synthesized only in primate placenta and suggest that its expression is regulated, in part, by gene methylation.