Carboxy-terminal determinants of intracellular protein degradation.

Using the amino-terminal domain of lambda repressor as a model system, we show that residues in an unstructured region at the extreme carboxyl terminus of the protein are important for determining its proteolytic susceptibility in Escherichia coli. Nonpolar amino acids are destabilizing when placed at the 5 carboxy-terminal residue positions, whereas charged and polar residues are stabilizing. The stabilizing effect of a single charged residue is greatest when it is at the terminal position and diminishes with increasing distance from the carboxyl terminus. The position of destabilizing sequences with respect to the free carboxyl terminus is important for their effect, but their distance from the folded portion of the protein is not important. Specific degradation of proteins with nonpolar carboxyl termini has been reconstituted in vitro using a partially pure, soluble fraction. This degradation is not ATP-dependent. Moreover, amino-terminal domain variants with nonpolar carboxy-terminal residues are still rapidly degraded in strains that are deficient in proteolysis of abnormal proteins. These data suggest that the degradation of amino-terminal domain variants with nonpolar carboxy-terminal residues involves proteolytic components distinct from those known to be important for the turnover of unfolded proteins in E. coli.