Figure 2.

Application of the P1 aspraginyl peptide vinyl sulfones to the proteasome from crude cellular extracts. (A) Direct labeling of the proteasome's multiple active sites in NIH 3T3 and EL-4 cells using two labels that differ only in the P1 residue. Change from P1 leucine to P1 aspragine leads to an increased labeling of the Z and MECL-1 subunits. (B) A typical competition profile obtained from incubation of crude EL-4 cell extracts with the complete set of P2 scanning sublibraries followed by labeling with ¹²⁵I-NIP-LLN-VS. The control, nontreated lane is indicated at the far left, and the identity of the constant amino acid for each sublibrary is shown along with its side-chain structure at the top of the gel.