A degradation signal located in the C‐terminus of p21 WAF1/CIP1 is a binding site for the C8 α‐subunit of the 20S proteasome

The cyclin‐dependent kinase inhibitor p21 WAF1/CIP1 is a key regulator of cell‐cycle progression and its expression is tightly regulated at the level of transcription and by proteasome‐dependent proteolysis. The turnover of p21 WAF1/CIP1 by proteasomes does not always require the ubiquitylation of p21 WAF1/CIP1 suggesting that there could be an alternative pathway into the proteasome. Here we show that the C8 α‐subunit of the 20S proteasome interacts with the C‐terminus of p21 WAF1/CIP1 and mediates the degradation of p21 WAF1/CIP1 . A small deletion in this region that disrupts binding to C8 increased the half‐life of p21 WAF1/CIP1 expressed in vivo . In contrast a deletion that increased the affinity between C8 and p21 WAF1/CIP1 significantly reduced the stability of the latter. These data suggest that interaction with a 20S proteasome α‐subunit is a critical determinant of p21 WAF1/CIP1 turn‐over and show how non‐ubiquitylated molecules might bypass the 19S regulator of the proteasome and become targeted directly to the 20S, core protease. Consistent with this, p21 WAF1/CIP1 was degraded rapidly by purified 20S proteasomes in a manner that was dependent on the C8‐interaction domain.