Essential role of CD8 palmitoylation in CD8 coreceptor function

To investigate the molecular basis that makes heterodimeric CD8 alpha beta a more efficient coreceptor than homodimeric CD8 alpha alpha, we used various CDS transfectants of T1.4 T cell hybridomas, which are specific for H-2K(d), and a photoreactive derivative of the Plasmodium berghei circumsporozoite peptide PbCS 252-260 (SYIPSAEKI). We demonstrate that CD8 is palmitoylated at the cytoplasmic tail of CD8 beta and that this allows partitioning of CD8 alpha beta, but not of CD8 alpha alpha, in lipid rafts. Localization of CD8 in rafts is crucial for its coreceptor function. First, association of CD8 with the src kinase p56(lck) takes place nearly exclusively in rafts, mainly due to increased concentration of both components in this compartment. Deletion of the cytoplasmic domain of CD8 beta abrogated localization of CD8 in rafts and association with p56(lck). Second, CD8-mediated cross-linking of p56(lck) by multimeric K-d-peptide complexes or by anti-CD8 Ab results in p56(lck) activation in rafts, from which the abundant phosphatase CD45 is excluded. Third, CD8-associated activated p56(lck) phosphorylates CD3 xi in rafts and hence induces TCR signaling and T cell activation. This study shows that palmitoylation of CD8 beta is required for efficient CD8 coreceptor function, mainly because it dramatically increases CD8 association with p56(lck) and CD8-mediated activation of p56(lck) in lipid rafts.