Cloning of ACP33 as a novel intracellular ligand of CD4.
CD4 recruitment to T cell receptor (TCR)-peptide-major histocompatibility class II complexes is required for stabilization of low affinity antigen recognition by T lymphocytes. The cytoplasmic portion of CD4 is thought to amplify TCR-initiated signal transduction via its association with the protein tyrosine kinase p56(lck). Here we describe a novel functional determinant in the cytosolic tail of CD4 that inhibits TCR-induced T cell activation. Deletion of two conserved hydrophobic amino acids from the CD4 carboxyl terminus resulted in a pronounced enhancement of CD4-mediated T cell costimulation. This effect was observed in the presence or absence of p56(lck), implying involvement of alternative cytosolic ligands of CD4. A two-hybrid screen with the intracellular portion of CD4 identified a previously unknown 33-kDa protein, ACP33 (acidic cluster protein 33), as a novel intracellular binding partner of CD4. Since interaction with ACP33 is abolished by deletion of the hydrophobic CD4 C-terminal amino acids mediating repression of T cell activation, we propose that ACP33 modulates the stimulatory activity of CD4. Furthermore, we demonstrate that interaction with CD4 is mediated by the noncatalytic alpha/beta hydrolase fold domain of ACP33. This suggests a previously unrecognized function for alpha/beta hydrolase fold domains as a peptide binding module mediating protein-protein interactions.
Zeitlmann
L
L
Laboratorium für Molekulare Biologie-Genzentrum der Universität München, and the GSF-Institut für Molekulare Immunologie, D-81377 München, Germany.
Sirim
P
P
Kremmer
E
E
Kolanus
W
W
D048868:Adaptor Proteins, Signal Transducing; D000595:Amino Acid Sequence; D000818:Animals; D015704:CD4 Antigens / Q000378:metabolism*; D019556:COS Cells; D002352:Carrier Proteins / Q000737:chemistry / Q000235:genetics* / Q000378:metabolism; D002460:Cell Line; D003001:Cloning, Molecular; D011992:Endosomes / Q000378:metabolism; D006056:Golgi Apparatus / Q000378:metabolism; D006801:Humans; D008024:Ligands; D008213:Lymphocyte Activation; D019860:Lymphocyte Specific Protein Tyrosine Kinase p56(lck) / Q000378:metabolism; D008969:Molecular Sequence Data; D009154:Mutation; D010641:Phenotype; D011485:Protein Binding; D017386:Sequence Homology, Amino Acid; D013347:Subcellular Fractions / Q000378:metabolism; D013601:T-Lymphocytes / Q000276:immunology
D048868:Adaptor Proteins, Signal Transducing; D015704:CD4 Antigens; D002352:Carrier Proteins; D008024:Ligands; C424703:SPG21 protein, human; D019860:Lymphocyte Specific Protein Tyrosine Kinase p56(lck)