T cell antigen CD28 binds to the GRB-2/SOS complex, regulators of p21ras.
The T cell molecule CD28 provides a co-stimulatory signal that is required for T cell proliferation, and has been implicated in the control of T cell anergy. An important clue to the signaling mechanism of CD28 is the finding that CD28 can bind to phosphatidylinositol 3-kinase (PI 3-kinase) by means of a cytoplasmic phospho-YMNM (pYMNM) motif. A remaining issue concerns whether CD28 can recruit other intracellular signaling molecules. In this study, we show that CD28 uses the same pYMNM motif to recruit a second intracellular protein, GRB-2. CD28-associated GRB-2, as detected by anti-GRB-2 immunoblotting, was found in human peripheral T cells, HPB-ALL and Jurkat cells. As in the case of PI3-kinase, antibody-induced cross-linking of CD28 induces a time-dependent recruitment of GRB-2. Likewise, mutation of the pY-191 residue within the pYMNM motif reduces GRB-2 binding. Peptide binding studies show that the SH2 domain of GRB-2 binds to the pYMNM motif with an affinity comparable to GRB-2/SHC, but some 10- to 100-fold lower than the CD28/PI 3-kinase. Despite this, CD28/GRB-2 and CD28/PI 3-kinase complexes are found to co-exist in peripheral T cells. Finally, immunoblotting shows that CD28 also associates with the gene product of the human homolog of the Drosophila Son of sevenless gene (SOS), a GRB-2-complexed guanine nucleotide exchange factor responsible for converting p21ras to a GTP-bound active state. CD28-associated GRB2/SOS is likely to serve an important link in the regulation of p21ras and lymphokine expression mediated by CD28.
Schneider
H
H
Division of Tumor Immunology, Dana-Farber Cancer Institute, Boston, MA 02115, USA.
Cai
Y C
YC
Prasad
K V
KV
Shoelson
S E
SE
Rudd
C E
CE
D048868:Adaptor Proteins, Signal Transducing; D000595:Amino Acid Sequence; D001665:Binding Sites / Q000235:genetics; D018106:CD28 Antigens / Q000235:genetics / Q000378:metabolism*; D051380:GRB2 Adaptor Protein; D006801:Humans; D008565:Membrane Proteins / Q000378:metabolism*; D008969:Molecular Sequence Data; D009154:Mutation; D011506:Proteins / Q000378:metabolism*; D016283:Proto-Oncogene Proteins p21(ras) / Q000378:metabolism; D015290:Second Messenger Systems; D017421:Sequence Analysis; D015398:Signal Transduction; D020664:Son of Sevenless Proteins; D013601:T-Lymphocytes / Q000276:immunology / Q000378:metabolism*; D014407:Tumor Cells, Cultured
D048868:Adaptor Proteins, Signal Transducing; D018106:CD28 Antigens; D051380:GRB2 Adaptor Protein; C495948:GRB2 protein, human; D008565:Membrane Proteins; D011506:Proteins; D020664:Son of Sevenless Proteins; C501009:HRAS protein, human; D016283:Proto-Oncogene Proteins p21(ras)