Folding pathway mediated by an intramolecular chaperone: intrinsically unstructured propeptide modulates stochastic activation of subtilisin.

Several secreted proteases are synthesized with N-terminal propeptides that function as intramolecular chaperones (IMCs) and direct the folding of proteases to their native functional states. Using subtilisin E as our model system, we had earlier established that (i) release and degradation of the IMC from its complex with the protease upon completion of folding is the rate-determining step to protease maturation and, (ii) IMC of SbtE is an extremely charged, intrinsically unstructured polypeptide that adopts an alpha-beta structure only in the presence of the protease. Here, we explore the mechanism of IMC release and the intricate relationship between IMC structure and protease activation. We establish that the release of the first IMC from its protease domain is a non-deterministic event that subsequently triggers an activation cascade through trans-proteolysis. By in silico simulation of the protease maturation pathway through application of stochastic algorithms, we further analyze the sub-stages of the release step. Our work shows that modulating the structure of the IMC domain through external solvent conditions can vary both the time and randomness of protease activation. This behavior of the protease can be correlated to varying the release-rebinding equilibrium of IMC, through simulation. Thus, a delicate balance underlies IMC structure, release, and protease activation. Proteases are ubiquitous enzymes crucial for fundamental cellular processes and require deterministic activation mechanisms. Our work on SbtE establishes that through selection of an intrinsically unstructured IMC domain, nature appears to have selected for a viable deterministic handle that controls a fundamentally random event. While this outlines an important mechanism for regulation of protease activation, it also provides a unique approach to maintain industrially viable subtilisins in extremely stable states that can be activated at will.