Sequence determinants of C-terminal substrate recognition by the Tsp protease

Cytochrome b₅₆₂ is not cleaved by the tail-specific protease Tsp in vitro or in the periplasm of Escherichia coli but becomes a good substrate when the C-terminal sequence WVAAA is added. Following randomization of the final three residue positions of this substrate, 54 different mutants with single residue substitutions were recovered. The steady-state expression levels of cytochrome variants bearing these mutant tails were similar in an E. coli strain deleted for the tsp gene but differed markedly in a strain containing Tsp. Wild-type cytochrome b₅₆₂ and seven variants, displaying a range of intracellular expression levels, were purified. These proteins were found to have the same T(m) values in thermal denaturation experiments but to be cleaved by Tsp at rates differing by as much as 30-fold. Overall, the rates of Tsp cleavage of these proteins in vitro correlate with their rates of cleavage in vivo as determined by pulse-chase experiments. These results indicate that the C-terminal sequence of the cytochrome-b₅₆₂ variants is important in determining their proteolytic fate in the cell and show that this degradation is mediated predominantly by Tsp. There are different selectivity rules at each of the three C-terminal positions. The identity of the C-terminal residue of the substrate, where small, uncharged residues (Ala, Cys, Ser, Thr, Val) are preferred, is most important in determining the rate of substrate cleavage by Tsp. Non-polar residues are also preferred at the second and third positions, but larger and more hydrophobic side chains are also acceptable at these positions in good substrates.