Identification of SAP as a CTLA-4 Binding Molecule: a Role of SAP in CTLA-4 Signaling Proposed.
- Author:
Kyung Mi LEE
1
Author Information
- Publication Type:Original Article
- Keywords: CTLA-4; SAP; SHP-2; tyrosine kinase; transfection; T cells
- MeSH: Autoradiography; Binding Sites; Clone Cells; Immunoblotting; Immunoprecipitation; Phosphorylation; Protein Tyrosine Phosphatase, Non-Receptor Type 11; Protein-Tyrosine Kinases; Receptors, Antigen, T-Cell; T-Lymphocytes; Transfection; Tyrosine
- From:Immune Network 2002;2(2):72-78
- CountryRepublic of Korea
- Language:English
- Abstract: BACKGROUND: The precise mechanism by which CTLA-4 regulates T cell immune responses is still not fully understood. Previously we proposed that CTLA-4 could downregulate T cell function by modulating a signaling cascade initiated from the T cell receptor complex. The evidence for this notion comes from our findings that CTLA-4 associated with the T cell receptor zeta (TCR zeta) chain, and hence regulated TCR zeta phosphorylation by co-associated SHP-2 tyrosine phosphatase (1). In this report, we investigated whether any other signaling molecules could be involved in the CTLA-4 signaling pathway. METHODS: We have taken biochemical approaches, such as immunoprecipitation followed by autoradiography or immunoblotting, to identify the molecules associated with CTLA-4. To perform these assays, we used activated primary T cells and ectopically transfected 293 cells. Various truncation mutants of CTLA-4 were used to map the interaction site on CTLA-4. RESULTS: We found that in addition to TCR zeta and SHP-2, a recently cloned small adaptor molecule, SAP (SLAM-associated protein), was also able to associate with CTLA-4. We identified the domain of SAP association in CTLA-4 being a motif involving GVYVKM. This motif has been previously found to bind SHP-2 through its phosphorylated tyrosine interaction with SH-2 domain of SHP-2. Indeed, co-expression of SAP and SHP-2 reduced their binding to CTLA-4 significantly, suggesting that SAP and SHP-2 compete for the common binding site, GVYVKM. Thus, by blocking SHP-2 recruitment SAP could function as a negative regulator of CTLA-4. CONCLUSION: Taken together, our data suggest the existence of complicate signaling cascade in regulating CTLA-4 function, and further provide evidence that SAP can act either as a positive or negative regulator depending on the nature of the associating receptors.
