SaOS2 cells were transfected with the respective siRNAs and then with the respective expression plasmids for VSVG-GFP, VSVG-Myc, VSVG-MT1-MMP, or VSVG-KDELR-Myc with or without expression plasmid for sr-IFT20 or IFT20. To examine the ER-to-cell surface transport, VSVG-GFP-transfected SaOS2 cells were incubated overnight at 40 °C in DMEM containing 1% FBS to accumulate VSVG-GFP at the ER, and then shifted to DMEM containing 1% FBS and 0.1 mg/ml cycloheximide pre-warmed at 32 °C to allow transport through the Golgi. After 30 or 60 min in culture, cell surface proteins were biotinylated with 0.5 mg/ml Sulfo-NHS-ss-biotin (Thermo) in Dulbecco’s PBS (DPBS) for 30 min at 4 °C and quenched with 50 mM NH4Cl in DPBS for 10 min at 4 °C. After washing with ice-cold DPBS, cells were solubilized with Triton X-100 lysis buffer [25 mM Tris-HCl (pH7.5), 150 mM NaCl, 5 mM EDTA, 1% (v/v) Triton X-100, 0.4% (w/v) sodium deoxycholate, 10 µg/ml leupeptin, 10 µg/ml aprotinin, 0.25 mM pAPMSF]. Biotinylated proteins were affinity-purified with streptavidin-Sepharose beads and subjected to SDS-PAGE followed Western blot analyses. ER-to-cis-Golgi and intra-Golgi transport of VSVG and VSVG-MT1-MMP and retrograde transport of VSVG-KDELR have been described previously48 (link).
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