Figure 6
BH4 positively regulates the activity of the proteasome. Posttranslational modification of proteasome targets by NO. (A) Cysteine residues within subunits of the 26S proteasome are profoundly modified in a BH4 dependent manner. (B–D) PSMC6, RPN2 and RPN5 subunits are all demonstrated to be altered at specific sites, where the Cys side chains are shown in green. Human models have been presented. (E) sEnd.1 endothelial cells were exposed to GCH-specific, or NS control siRNA, and proteasome activity was assessed using a Promega proteasome activity assay as in the Experimental Procedures. Knockdown of BH4 levels, and subsequent NO/redox imbalance using siRNA, significantly inhibited proteasome activity (n = 4, *P < 0.05). (F) Nitrosation sites in PSMB2 (shown in Green) are only visible upon processing of the protein where cleavage at 53Thr and 78Thr occurs during proteasome maturation as shown in red and blue, respectively (G,H).
