aLight- (L) or heavy-labeled (H) cells were treated with MG132 (MG) for 2 or 6 hr, as indicated. Equal amounts of light- and heavy-labeled extracts were mixed, and endogenous VAPB was immunoprecipitated using specific antibodies. The light/heavy SILAC ratios (L/H) determined by mass spectrometry are indicated. L + MG indicates that the light-labeled samples were treated with MG132. Proteins whose interaction with VAPB is stimulated by proteasome inhibition accumulate in these samples, resulting in L/H ratios higher than 1. H + MG indicates that the heavy-labeled samples were treated with MG132. Protein accumulation in the heavy-labeled samples results in L/H ratios lower than 1. The L/H for proteins that are not affected by proteasome inhibition will be close to 1. The numbers in parenthesis represent the number of spectra analyzed for each sample.