Evidence for a novel RasGAP-associated protein of 105 kDa in both mature trophoblasts and differentiating choriocarcinoma cells.

A novel tyrosine-phosphorylated, RasGAP-associated protein of 105 kDa (p105) is found in normal human term placental trophoblasts, as well as in JEG-3 human choriocarcinoma cells induced to differentiate by okadaic acid (OA). This p105 RasGAP-associated protein is distinct from other RasGAP-associated proteins described so far, none of which has either a molecular size close to p105 or a trophoblastic cell origin. The p105 appears, accompanied by p120 and p100 RasGAP expression, after OA treatment of JEG-3 cells but is almost undetectable in the absence of stimulation. Moreover, the p105 is the first discovered RasGAP-associated protein bound to p100 RasGAP. The natural occurrence of the p105 in normal mature trophoblasts isolated from human term placenta suggests that it may be linked to the differentiation state of human trophoblasts. Hence, this p105 RasGAP-associated protein might be considered a marker of human trophoblast differentiation.