Immunoprecipitation assay
For immunoprecipitation assay, 4× hemagglutinin (HA) or 3× Flag tagged cDNA fragment encoding full-length mr-s (full-HA and Flag-mrs), a cDNA fragment encoding amino acids 1–400 (ΔSAM-HA and Flag-ΔSAM), and a cDNA fragment encoding amino acids 400–542 (Flag-SAM) were subcloned into the pcDNA3 (Invitrogen) expression vector. We also constructed two site-directed mutants, Flag-W404A and Flag-G453A, using PCR. Each of these mutations was also introduced into Flag-mr-s. We transfected HEK293T cells with 5 μg of plasmid DNA per 6 cm dish by calcium phosphate method. Approximately 48 hr after transfection, cells were harvested in immunoprecipitation buffer (50 mM Tris-HCl [pH 7.5], 1 mM EDTA, 2 mM MgCl2, 150 mM NaCl, 1 mM phenylmethylsulfonyl fluoride [Wako], 1% Nonidet P-40, 10% glycerol) in the presence of protease inhibitor cocktail tablets (Roche). For each reaction, 1 mg of cell lysate was mixed with 0.5 μg of anti-Flag antibody (SIGMA, F3165) and 15 μl of protein G-Sepharose (Amersham) on a rotating wheel at 4°C for 2 hrs. Protein concentration was determined by the BCA protein assay system (Pierce). The beads were then washed three times with immunoprecipitation buffer and followed by three times with wash buffer (50 mM Tris-HCl [pH 7.5], 150 mM NaCl, 20 mM MgCl2). Proteins were then boiled for 5 min in SDS sample buffer (1% SDS, 1 mM Tris [pH 6.8], 40 mM DTT, 4% glycerol, 0.01% pyronine Y). The supernatants were fractionated by sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) and transferred to the nitrocellulose membrane (Trans-Blot Transfer Medium, Bio-Rad). Western blotting was performed with rabbit polyclonal anti-HA antibody (Santa Cruz, #sc-805). Signals were detected with horseradish peroxidase- conjugated goat anti-rabbit IgG and ECL plus Western Blotting Detection System (Amersham).