Figure 6. Model for Detecting MHC I-Bound Antigentic Precursors.

(A) The presence of a proline residue among the N-terminal flanking residues inhibits ERAAP’s ability to generate the final QL9-L^d complex. The ERAAP-TAP double-deficient fibroblasts or COS cells were cotransfected with ER-targeted, N-terminally extended QL9 precursors without (ES-X6[QL9]) or with (ES-X3-EPK[QL9]), a proline residue (P), the L^d MHC, and either ERAAP or vector alone. The presence of the QL9-L^d complex on the cell surface was measured with 2CZ T cells.

(B) The TAP inhibitor ICP47 blocks presentation of cytoplasmic but not ER-targeted QL9 precursors in COS cells. Simian COS cells were cotransfected with constructs encoding cytoplasmic (MK[QL9]) or ER-targeted (ES-X6[QL9], ES-X3-EPK[QL9]) precursors, and the indicated cDNAs encoding L^d, ICP47, or vector alone. The presence of the QL9-L^d complex on the cell surface was measured with 2CZ T cells. Data are representative of at least two independent experiments. The similar results were confirmed by using either ES-X3-LPK[QL9] or -QPK[QL9] instead of -EPK[QL9] (data not shown).