The folding landscape of Streptomyces griseus protease B reveals the energetic costs and benefits associated with evolving kinetic stability

Like most extracellular bacterial proteases, Streptomyces griseus protease B (SGPB) and alpha-lytic protease (alphaLP) are synthesized with covalently attached pro regions necessary for their folding. In this article, we characterize the folding free energy landscape of SGPB and compare it to the folding landscapes of alphaLP and trypsin, a mammalian homolog that folds independently of its zymogen peptide. In contrast to the thermodynamically stable native state of trypsin, SGPB and alphaLP fold to native states that are thermodynamically marginally stable or unstable, respectively. Instead, their apparent stability arises kinetically, from unfolding free energy barriers that are both large and highly cooperative. The unique unfolding transitions of SGPB and alphaLP extend their functional lifetimes under highly degradatory conditions beyond that seen for trypsin; however, the penalty for evolving kinetic stability is remarkably large in that each factor of 2.4-8 in protease resistance is accompanied by a cost of ~10(5) in the spontaneous folding rate and ~5-9 kcal/mole in thermodynamic stability. These penalties have been overcome by the coevolution of increasingly effective pro regions to facilitate folding. Despite these costs, kinetic stability appears to be a potent mechanism for developing native-state properties that maximize protease longevity.