Targets of B lymphocyte antigen receptor signal transduction include the p21ras GTPase-activating protein (GAP) and two GAP-associated proteins.
Cross-linking membrane Ig (mIg) on B cells stimulates tyrosine phosphorylation of proteins involved in signal transduction including the mIg-associated proteins Ig-alpha and Ig-beta, the tyrosine kinases p53/p56lyn, p55blk, p59fyn, and PTK72, phosphatidylinositol 3-kinase, phospholipase C gamma 1 and gamma 2, and the mitogen-activated protein kinase. We now show that the p21ras GTPase-activating protein (GAP) is also a substrate for mIg-activated tyrosine kinases. p21ras is a key regulator of cell growth and GAP may act as both a regulator of p21ras activity and as a downstream effector of p21ras. We found that mIg cross-linking caused a rapid increase in tyrosine phosphorylation of GAP in the immature B cell line WEHI-231, the mature B cell lines BAL 17 and Daudi, and the IgG-bearing B cell line A20. In fibroblasts, tyrosine kinase activation causes GAP to associate with two other tyrosine-phosphorylated proteins, p62 and p190, which have homologies to an RNA-binding protein and a transcriptional repressor, respectively. Similarly, mlg cross-linking induced the association of GAP with a 62-kDa tyrosine-phosphorylated protein in BAL 17, WEHI-231, and Daudi cells. Anti-Ig treatment also increased the amount of a 190-kDa tyrosine-phosphorylated protein associated with GAP in WEHI-231 and Daudi cells. After separation by SDS-PAGE and transfer to nitrocellulose, the tyrosine-phosphorylated p62 and p190 present in anti-GAP immunoprecipitates from B cells were capable of binding radiolabeled recombinant GAP, as previously reported for the GAP-associated p62 and p190 from fibroblasts. The amount of p62 that could be detected in this way after immunoprecipitation with antiphosphotyrosine antibodies was much greater from anti-IgM-treated BAL 17 cells than from unstimulated BAL 17 cells. This probably reflects anti-Ig-induced tyrosine phosphorylation of p62. In any case, GAP, p62, and/or p190 may be involved in signal transduction by mIg in B cells.
Gold
M R
MR
George Williams Hooper Foundation, San Francisco, CA 94143.
Crowley
M T
MT
Martin
G A
GA
McCormick
F
F
DeFranco
A L
AL
D000818:Animals; D004268:DNA-Binding Proteins / Q000378:metabolism*; D020690:GTPase-Activating Proteins; D020662:Guanine Nucleotide Exchange Factors; D006801:Humans; D051379:Mice; D009687:Nuclear Proteins / Q000378:metabolism*; D010750:Phosphoproteins / Q000378:metabolism*; D010766:Phosphorylation; D011506:Proteins / Q000378:metabolism*; D016601:RNA-Binding Proteins / Q000378:metabolism*; D011947:Receptors, Antigen, B-Cell / Q000502:physiology*; D012097:Repressor Proteins; D015398:Signal Transduction; D014443:Tyrosine / Q000378:metabolism
C074560:ARHGAP35 protein, human; C513041:Arhgap35 protein, mouse; D004268:DNA-Binding Proteins; C489560:DOK1 protein, human; C489561:Dok1 protein, mouse; C074561:GAP-associated protein p62; D020690:GTPase-Activating Proteins; D020662:Guanine Nucleotide Exchange Factors; D009687:Nuclear Proteins; D010750:Phosphoproteins; D011506:Proteins; D016601:RNA-Binding Proteins; D011947:Receptors, Antigen, B-Cell; D012097:Repressor Proteins; D014443:Tyrosine