Overlapping recognition determinants within the ssrA degradation tag allowmodulation of proteolysis

The ssrA tag, an 11-aa peptide added to the C terminus of proteins stalled during translation, targets proteins for degradation by ClpXP and ClpAP. Mu tational analysis of the ssrA tag reveals independent, but overlapping dete rminants for its interactions with ClpX, ClpA, and SspB, a specificity-enha ncing factor for ClpX. ClpX interacts with residues 9-11 at the C terminus of the tag, whereas OpA recognizes positions 8-10 in addition to residues 1 -2 at the N terminus. SspB interacts with residues 1-4 and 7, N-terminal to the ClpX-binding determinants, but overlapping the ClpA determinants. As a result, SspB and ClpX work together to recognize ssrA-tagged substrates ef ficiently, whereas SspB inhibits recognition of these substrates by ClpA. T hus, dissection of the recognition signals within the ssrA tag provides ins ight into how multiple proteins function in concert to modulate proteolysis .