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Plasticity and Rigidity in Adaptor Protein-2-Mediated Internalization of the TCR:CD3 Complex¹

Department of Immunology, St. Jude Children’s Research Hospital, Memphis, TN 38105

Many cell surface proteins are internalized via dileucine- or tyrosine-based motifs within their cytoplasmic domains by the heterotetrameric adaptor protein complex, AP-2. In this study we have examined how AP-2 mediates internalization of large cell surface receptors, such as the eight-chain TCR:CD3 complex. Although most receptors have a single signal that drives internalization, the TCR complex has two (D/E)xxxL(L/I) motifs and 20 YxxØ motifs. Using 293T cells, we show that AP-2 is completely dependent on both signals to mediate TCR internalization, because deletion of either completely blocks this process. Significant plasticity and redundancy were observed in the use of the YxxØ motifs, with a clear hierarchy in their use (CD3 > CD3 CD3 >> CD3). Remarkably, a single, membrane-distal YxxØ motif in CD3 could mediate 75% of receptor internalization, whereas its removal only reduced internalization by 20%. In contrast, significant rigidity was observed in use of the (D/E)xxxL(L/I) motif in CD3. This was due to an absolute requirement for the position of this signal in the context of the TCR complex and for a highly conserved lysine residue, K128, which is not present in CD3. These contrasting requirements suggest a general principle by which AP-2 may mediate the internalization of large, multichain complexes.

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