THE CLPXP AND CLPAP PROTEASES DEGRADE PROTEINS WITH CARBOXY-TERMINAL PEPTIDE TAILS ADDED BY THE SSRA-TAGGING SYSTEM

Interruption of translation in Escherichia coli can lead to the additi on of an Ii-residue carboxy-terminal peptide tail to the nascent chain . This modification is mediated by SsrA RNA (also called 10Sa RNA and tmRNA) and marks the tagged polypeptide for proteolysis. Degradation i n vivo of lambda repressor amino-terminal domain variants bearing this carboxy-terminal SsrA peptide tag is shown here to depend on the cyto plasmic proteases ClpXP and ClpAP. Degradation in vitro of SsrA-tagged substrates was reproduced with purified components and required a sub strate with a wild-type SsrA tail, the presence of both ClpP and eithe r ClpA or ClpX, and ATP. Clp-dependent proteolysis accounts for most d egradation of SsrA-tagged amino-domain substrates at 32 degrees C, but additional proteases contribute to the degradation of some of these S srA-tagged substrates at 39 degrees C. The existence of multiple cytop lasmic proteases that function in SsrA quality-control surveillance su ggests that the SsrA tag is designed to serve as a relatively promiscu ous signal for proteolysis. Having diverse degradation systems able to recognize this tag may increase degradation capacity, permit degradat ion of a wide variety of different tagged proteins, or allow SsrA-tagg ed proteins to be degraded under different growth conditions.