A specificity-enhancing factor for the ClpXP degradation machine

Events that stall bacterial protein, synthesis activate the ssrA-tagging ma chinery, resulting in resumption of translation and addition of an 11-resid ue peptide to the carboxyl terminus of the nascent chain. This ssrA-encoded peptide tag marks the incomplete protein for degradation by the energy-dep endent ClpXP protease. Here, a ribosome-associated protein, SspB, was found to bind specifically to ssrA-tagged proteins and to enhance recognition of these proteins by ClpXP. Cells with an sspB mutation are defective in degr ading ssrA-tagged proteins, demonstrating that SspB is a specificity-enhanc ing factor for ClpXP that controls substrate choice.