CLONING OF A GRB2 ISOFORM WITH APOPTOTIC PROPERTIES

Growth factor receptor-bound protein 2 (Grb2) links tyrosine-phosphory lated proteins to a guanine nucleotide releasing factor of the son of sevenless (Sos) class by attaching to the former by its Src homology 2 (SH2) moiety and to the latter by its SH3 domains. An isoform of grb2 complementary DNA (cDNA) was cloned that has a deletion in the SH2 do main. The protein encoded by this cDNA, Grb3-3, did not bind to phosph orylated epidermal growth factor receptor (EGFR) but retained function al SH3 domains and inhibited EGF-induced transactivation of a Ras-resp onsive element. The messenger RNA encoding Grb3-3 was expressed in hig h amounts in the thymus of rats at an age when massive negative select ion of thymocytes occurs. Microinjection of Grb3-3 into Swiss 3T3 fibr oblasts induced apoptosis. These findings indicate that Grb3-3, by act ing as a dominant negative protein over Grb2 and by suppressing prolif erative signals, may trigger active programmed cell death.