Figure 3.

The peptide vinyl sulfone, ¹²⁵I-NIP-L₃VS, labels multiple β subunits of the proteasome in total cell extracts and in purified proteasome preparations. Total cell extracts (T) and purified preparations of proteasomes (P) were incubated with ¹²⁵I-NIP-L₃-VS (1.8 × 10⁴ Bq/ml) at 37°C for 2 hr, resolved on a 12.5% polyacrylamide gel and visualized by silver staining, by autoradiography or by two-dimensional isoelectric focusing SDS/PAGE for rabbit muscle (A), S. cerevisiae (B), or T. acidophilum (C). For T. acidophilum, recombinant β subunits containing an NH₂-terminal threonine, serine, or alanine were compared, as indicated.