*2.6. Antibodies*

Antibodies used for immunoblotting: mouse anti-TAP1 monoclonal antibody MAb 143.5 (kindly provided by R. Tampé, Institute of Biochemistry, The Johann Wolfgang Goethe University, Frankfurt, Germany); mouse anti-TAP2 MAb 435.3 (a kind gift from P. van Endert, INSERM U25, Institute Necker, Paris, France); rabbit anti-TAP1 (Enzo Life Sciences, Farmingdale, NY, USA); rat anti-GFP 3H9 (Chromotek, Planegg, Germany); mouse anti-myc tag 9B11 (Cell Signaling, Danvers, MA, USA); rabbit anti-β-actin (Novus Biologicals, Centennial, CO, USA); rabbit anti-β-catenin (Santa Cruz Biotechnology, Dallas, TX, USA); rabbit antibodies (H11) against a synthetic peptide derived from the N-terminal domain of BoHV-1 UL49.5 [26] and mouse anti-OctA (FLAG) G-8 (Santa Cruz Biotechnology); and mouse anti-HC10 [19] and rabbit anti-ERp57 H-220 (Santa Cruz Biotechnology). Probes used for immunofluorescence: Alexa 633-conjugated concanavalin A (ConA) (Thermo Scientific). Antibodies used for flow cytometry: mouse anti-MHC I W6/32 (Novus Biologicals); mouse anti-NGFR (Sigma-Aldrich); and Alexa 633-conjugated goa<sup>t</sup> anti-mouse IgG (Thermo Scientific).

### *2.7. Flow Cytometry*

Cell surface expression of MHC I was determined by indirect immunofluorescence using primary anti-MHC I antibodies (1:1000) and secondary antibodies (1:1000), all in phosphate bu ffered saline (PBS) bu ffer containing 1% bovine serum albumin and 0.02% sodium azide. For cell sorting, anti-NGFR antibodies (1:1000) and secondary antibodies were used. Cells were analyzed using a FACS Calibur flow cytometer (Becton Dickinson, Franklin Lakes, NJ, USA) and CellQuest software (version 5.2.1, Becton Dickinson)); for cell sorting, the sorting option of FACS Calibur was applied.

### *2.8. Immunoblotting and Immunoprecipitation*

For immunoblotting, the cells were lysed in Cell Lytic M bu ffer (Sigma-Aldrich); for immunoprecipitation, the cells were lysed in a bu ffer containing 1% digitonin (Merck), 50 mM Tris-HCl, pH 7.5, 5 mM MgCl2, and 150 mM NaCl. The bu ffers were supplemented with the cOmplete mini protease inhibitor cocktail (Roche, Basel, Switzerland). Cell lysates were analyzed by SDS-PAGE and immunoblotting as previously described [26] or incubated with GFP-Trap (Chromotek) according to the manufacturer's protocol to isolate protein complexes.

### *2.9. Peptide Transport Assay*

The peptide transport assay was performed as described before [26]. Briefly, the cells were permeabilized with 2 IU mL−<sup>1</sup> of Streptolysin O (Sigma-Aldrich) at 37 ◦C for 15 min. The cells (2 × 10<sup>6</sup> cells/sample) were subsequently incubated with 600 pmol of the fluorescein-conjugated synthetic peptide CVNKTERAY (JPT Peptide Technologies, Berlin, Germany) in the presence or absence of ATP (10 mM final concentration) at 37 ◦C for 10 min. Peptide translocation was terminated by adding 1 mL of ice-cold lysis buffer containing 1% Triton X-100. After 20 min of lysis, cell debris was removed by centrifugation, and supernatants were collected and incubated with 100 μL of concanavalin A (ConA)-Sepharose (Sigma-Aldrich) at 4 ◦C for 1 h to isolate the glycosylated peptides. After extensive washing of the beads, the peptides were eluted with elution buffer (500 mM mannopyranoside, 10 mM EDTA, 50 mM Tris-HCl, pH 8.0) by rigorous shaking for 1 h at 25 ◦C. Eluted peptides were separated from ConA by centrifugation at 12,000× *g*. The fluorescence intensity was measured using a fluorescence plate reader (EnVision, PerkinElmer, Waltham, MA, USA) with excitation and emission wavelengths of 485 nm and 530 nm, respectively.
