SAM68 IS A RAS-GAP-ASSOCIATED PROTEIN IN MITOSIS

Sam68 is the major tyrosine-phosphorylated and Src-associated protein in mitotic cells. Sam68 stimulates G1/S transition and this effect is dependent on the integrity of its RH domain (hnRNPK Homology) which co nfers nucleic acid binding properties. During mitosis, MATERIALS AND M ETHODS Sam68 undergoes tyrosine phosphorylation, which negatively regu lates its nucleic acid binding properties and mediates the interaction of Sam68 with critical SH2-containing signaling proteins such as Grb2 , PLC gamma 1 and Ras-GAP. However, the interaction of Ras-GAP with Sa m68 has been brought into question, based on the lack of co-immunoprec ipitation between Sam68 and Ras-GAP in interphase cells. Here we show that the choice of anti-Ras-GAP antibodies is critical for the detecti on of Ras-GAP/Sam68 complex formation, and that this interaction is sp ecific for G2/M transition in both NHI3T3 and Src-transformed cells. S uch data reinforce the importance of the interaction of Ras-GAP with R NA binding proteins during cell proliferation through its SH2 and SH3 domains. (C) 1998 Academic Press.