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Actin cytoskeleton dynamics critically regulate T cell activation. We found that the cytoplasmic adaptor HIP-55, a Src/Syk-kinases substrate and member of the drebrin/Abp1 family of actin-binding proteins, localized to the T cell-antigen-presenting cell (APC) contact site in an antigen-dependent manner. Using green fluorescent protein fusion proteins, both Src homology 3 (SH3) and actin binding domains were found necessary for recruitment at the T cell-APC interface. HIP-55 was not implicated in conjugate formation and actin polymerization but regulated distal signaling events through binding and activation of hematopoietic progenitor kinase 1 (HPK1), a germinal center kinase (GCK) family kinase involved in negative signaling in T cells. Using RNA interference and overexpression experiments, the HIP-55-HPK1 complex was found to negatively regulate nuclear factor of activated T cell (NFAT) activation by the T cell antigen receptor. Moreover, we show that HIP-55, which partly co-localized with early endocytic compartments, promoted both basal and ligand-dependent T cell receptor (TCR) down-modulation, resulting in a decreased TCR expression. SH3 and actin-depolymerizing factor homology domains were required for this function. As controls, the expression of CD28 and the glycosylphosphatidylinositol-linked protein CD59 was not affected by HIP-55 overexpression. These results suggest that, in addition to binding to HPK1, HIP-55 might negatively regulate TCR signaling through down-regulation of TCR expression. Our findings show that HIP-55 is a key novel component of the immunological synapse that modulates T cell activation by connecting actin cytoskeleton and TCRs to gene activation and endocytic processes.

Original publication




Journal article


J Biol Chem

Publication Date





15550 - 15560


Actins, Animals, Binding Sites, CD28 Antigens, CD3 Complex, CD4-Positive T-Lymphocytes, CD59 Antigens, Cell Separation, Cytoskeleton, DNA-Binding Proteins, Down-Regulation, Endocytosis, Enzyme Activation, Flow Cytometry, Green Fluorescent Proteins, Humans, Immunoblotting, Interleukin-2, Jurkat Cells, Ligands, Luciferases, Luminescent Proteins, Lymphocyte Activation, Mice, Microfilament Proteins, Microscopy, Confocal, Microscopy, Fluorescence, NFATC Transcription Factors, Nuclear Proteins, Plasmids, Protein Structure, Tertiary, Protein-Serine-Threonine Kinases, RNA Interference, Receptors, Antigen, T-Cell, Recombinant Fusion Proteins, Signal Transduction, Synapses, Time Factors, Transcription Factors, Transfection, src Homology Domains