CHARACTERIZATION OF FULL-LENGTH NEUROFIBROMIN - TUBULIN INHIBITS RAS GAP ACTIVITY

Full-length neurofibromin is a GTPase activating protein (GAP) for the Ras proto-oncogene product. Regulation of neurofibromin activity ther efore has important implications for cell growth. Neurofibromin co-pur ifies with tubulin when expressed in insect cells. The interaction bet ween neurofibromin and tubulin is sensitive to the microtubule depolym erizing agent colchicine. Neurofibromin GAP activity is inhibited even at low concentrations of tubulin. However, maximal inhibition of GAP activity is only approximately 70%, suggesting that the neurofibromin- tubulin complex retains residual GAP activity. This decreased activity is reflected by a 4-fold decrease in its affinity for Ras. A truncate d mutant of neurofibromin with reduced sensitivity to tubulin localize s some tubulin-binding determinants to an 80 residue segment immediate ly N-terminal to the GAP-related domain. Since tubulin is an abundant protein in eukaryotic cells, the tubulin-neurofibromin interaction may regulate the Ras signalling