SKAP55 modulates T cell antigen receptor-induced activation of the Ras-Erk-AP1 pathway by binding RasGRP1.
Src kinase-associated phosphoprotein of 55 kDa (SKAP55) is an adapter protein with an N-terminal region, a pleckstrin homology domain, a linker with tyrosine phosphorylation sites, and a C-terminal Src homology 3 domain. We report that overexpression of SKAP55 disrupts signaling from the TCR to the Ras-Erk-AP-1 pathway and transcription of the IL-2 gene in primary human T cells and in Jurkat T leukemia cells. In contrast, moderate overexpression of SKAP55 increased TCR-dependent AP-1 transcriptional activity, suggesting that high-level SKAP55 overexpression interfered with the assembly of functional signaling complexes required for TCR coupling to the Ras pathway. In support of this view, knock-down of SKAP55 by RNA interference resulted in decreased reporter gene activation and decreased ERK phosphorylation. In contrast, TCR-induced NF-kappaB activation was not affected. Since constitutively active forms of Ras or Raf-1 overcame the inhibitory effects of SKAP55 overexpression, we searched for a mechanism upstream of Ras and found that SKAP55 co-immunoprecipitated with the Ras activator RasGRP1. The binding of RasGRP1 to SKAP55 required the C-terminus of SKAP55 and was enhanced by tyrosine phosphorylation of SKAP55. These results suggest that SKAP55 modulates signal transduction from the TCR to Ras by binding to RasGRP1.
Program on Inflammatory Disease Research, Infectious and Inflammatory Disease Center, The Burnham Institute for Medical Research, 10901 North Torrey Pines Road, La Jolla, CA 92037, USA. karenakosco@yahoo.com
Karena A
KA
Kosco
Fabio
F
Cerignoli
Scott
S
Williams
Robert T
RT
Abraham
Tomas
T
Mustelin
D002460:Cell Line; D004268:DNA-Binding Proteins / Q000737:chemistry / Q000378:metabolism*; D004789:Enzyme Activation; D048049:Extracellular Signal-Regulated MAP Kinases / Q000378:metabolism*; D020868:Gene Silencing; D017930:Genes, Reporter; D020662:Guanine Nucleotide Exchange Factors / Q000737:chemistry / Q000378:metabolism*; D006801:Humans; D010750:Phosphoproteins / Q000737:chemistry / Q000172:deficiency / Q000378:metabolism*; D011485:Protein Binding; D025941:Protein Interaction Mapping; D017434:Protein Structure, Tertiary; D011948:Receptors, Antigen, T-Cell / Q000378:metabolism*; D015398:Signal Transduction; D013601:T-Lymphocytes / Q000201:enzymology; D018808:Transcription Factor AP-1 / Q000378:metabolism*; D018631:ras Proteins / Q000378:metabolism*
D004268:DNA-Binding Proteins; D020662:Guanine Nucleotide Exchange Factors; D010750:Phosphoproteins; C491790:RASGRP1 protein, human; D011948:Receptors, Antigen, T-Cell; C108336:SKAP1 protein, human; D018808:Transcription Factor AP-1; D048049:Extracellular Signal-Regulated MAP Kinases; D018631:ras Proteins