Escherichia coli stringent starvation protein B (SspB), is thought to enhance the specificity of degradation of tmRNA-tagged proteins by the ClpXP protease. The tmRNA tag, also known as ssrA, is an 11-aa peptide added to the C terminus of proteins stalled during translation, targets proteins for degradation by ClpXP and ClpAP. SspB a cytoplasmic protein that specifically binds to residues 1-4 and 7 of the tag. Binding of SspB enhances degradation of tagged proteins by ClpX, and masks sequence elements important for ClpA interactions, inhibiting degradation by ClpA [1]. However, more recent work has cast doubt on the importance of SspB in wild-type cells [2]. SspB is encoded in an operon whose synthesis is stimulated by carbon, amino acid, and phosphate starvation. SspB may play a special role during nutrient stress, for example by ensuring rapid degradation of the products of stalled translation, without causing a global increase in degradation of all ClpXP substrates [3].

1: Overlapping recognition determinants within the ssrA degradation tag allow modulation of proteolysis. Flynn JM, Levchenko I, Seidel M, Wickner SH, Sauer RT, Baker TA; Proc Natl Acad Sci U S A 2001;98:10584-10589. PMID:11535833

2: Screening for stabilization of proteins with a trans-translation signature in Escherichia coli selects for inactivation of the ClpXP protease. Bohn C, Binet E, Bouloc P; Mol Genet Genomics 2002;266:827-831. PMID:11810257

3: A specificity-enhancing factor for the ClpXP degradation machine. Levchenko I, Seidel M, Sauer RT, Baker TA; Science 2000;289:2354-2356. PMID:11009422