TI  - The SH2/SH3 domain-containing protein   <prot>GRB2</prot>   interacts with
      tyrosine-phosphorylated  <prot>IRS1</prot>  and  <prot>Shc</prot> : implications for insulin control of
      <prot>ras</prot> signalling.
PG  - 1929-36
AB  - <prot>GRB2</prot>, a small protein comprising one SH2 domain and two SH3 domains,
      represents the human homologue of the Caenorhabditis elegans protein,
      <prot>sem-5</prot>. Both <prot>GRB2</prot> and <prot>sem-5</prot> have been implicated in a highly conserved
      mechanism that regulates <prot>p21ras</prot> signalling by receptor tyrosine kinases.
      In this report we show that in response to <prot>insulin</prot>,   <prot>GRB2</prot>   forms a stable
      complex with two tyrosine-phosphorylated proteins. One protein is the
      major insulin receptor substrate  <prot>IRS-1</prot>  and the second is the SH2
      domain-containing oncogenic protein,  <prot>Shc</prot> . The interactions between <prot>GRB2</prot>
      and these two proteins require ligand activation of the <prot>insulin receptor</prot>
      and are mediated by the binding of the SH2 domain of   <prot>GRB2</prot>   to
      phosphotyrosines on both  <prot>IRS-1</prot>  and  <prot>Shc</prot> . Although   <prot>GRB2</prot>   associates with
       <prot>IRS-1</prot>  and  <prot>Shc</prot> , it is not tyrosine-phosphorylated after <prot>insulin</prot>
      stimulation, implying that <prot>GRB2</prot> is not a substrate for the insulin
      receptor. Furthermore, we have identified a short sequence motif (YV/IN)
      present in  <prot>IRS-1</prot> ,  <prot>EGFR</prot>  and  <prot>Shc</prot> , which specifically binds the SH2 domain of
         <prot>GRB2</prot>    with high affinity. Interestingly, both  <prot>GRB2</prot>  and
       <prot>phosphatidylinositol-3 (PI-3) kinase</prot>  can simultaneously bind distinct
      tyrosine phosphorylated regions on the same   <prot>IRS-1</prot>   molecule, suggesting a
      mechanism whereby <prot>IRS-1</prot> could provide the core for a large signalling
      complex. We propose a model whereby insulin stimulation leads to formation
      of multiple protein--protein interactions between   <prot>GRB2</prot>   and the two targets
       <prot>IRS-1</prot>  and  <prot>Shc</prot> . These interactions may play a crucial role in activation of
      <prot>p21ras</prot> and the control of downstream effector molecules.
AD  - New York University Medical Center, Department of Pharmacology, NY 10016.
