Cytolytic T lymphocyte-associated antigen-4 and the TCR zeta/CD3 complex, but not CD28, interact with clathrin adaptor complexes AP-1 and AP-2

The negative signaling receptor cytolytic T lymphocyte-associated Ag-4 (CTL A-4) resides primarily in intracellular compartments such as the Golgi appa ratus of T cells. However, little is known regarding the molecular mechanis ms that influence this accumulation. In this study, we demonstrate binding of the clathrin adaptor complex: AP-1 with the GVYVKM motif of the cytoplas mic domain of CTLA-4 Binding occurred primarily in the Golgi compartment of T cells, unlike with AP-2 binding that occurs mostly with cell surface CTL A-4. Although evidence was not found to implicate AP-1 binding in the reten tion of CTLA-4 in the Golgi, AP-1 appears to play a role in shuttling of ex cess receptor from the Golgi to the lysosomal compartments for degradation. In support of this, increased CTLA-4 synthesis resulted in an increase in CTLA-4/AP-1 binding and a concomitant increase in the appearance of CTLA-4 in the lysosomal compartment. At the same time, the level of intracellular receptor was maintained at a constant level, suggesting that CTLA-4/AP-1 bi nding represents one mechanism to ensure steady state levels of intracellul ar CTLA-4 in T cells. Finally, we demonstrate that the TCR zeta/CD3 complex (but not CD28) also binds to AP-1 and AP-2 complexes, thus providing a pos sible link between these two receptors in the regulation of T cell function .