To order to determine cellular ubiquitination levels, wildtype (WT) and UFD1s-deficent (MUT) cells were treated with 25 μM MG132 for 6 h. Cells were then lysed in UA buffer (8 M Urea, 100 mM Tris-HCl pH 8.5) on ice for 10 min. Cells were harvested, frozen in liquid nitrogen and stored at -80 ℃ for further ubiquitin (label-free)-modified proteome.