Folding pathway mediated by an intramolecular chaperone: propeptide release modulates activation precision of pro-subtilisin

Propeptides of several proteases directly catalyze the protein folding reaction. Uncatalyzed folding traps these proteases into inactive molten-globule-like conformers that switch into active enzymes only when their cognate propeptides are added in trans. Although tight binding and proteolytic susceptibility forces propeptides to function as single turnover catalysts, the significance of their inhibitory function and the mechanism of activation remain unclear. Using pro-subtilisin as a model, we establish that precursor activation is a highly coordinated process that involves synchronized folding, autoprocessing, propeptide release, and protease activation. Our results demonstrate that activation is controlled by release of the first free active protease molecule. This triggers an exponential cascade that selectively targets the inhibitory propeptide in the autoprocessed complex as its substrate. However, a mutant precursor that enhances propeptide release can drastically reduce the folding efficiency by altering the synergy between individual stages. Our results represent the first demonstration that propeptide release, not precursor folding, is the rate-determining step and provides the basis for the proposed model for precise spatial and temporal activation that allows proteases to function as regulators of biological function.

Functional analysis of the propeptides of subtilisin E and aqualysin I as intramolecular chaperones

Several proteases require propeptides for the correct folding of their own protease domain. We have recently found that the propeptide from a thermostable subtilisin homolog aqualysin I can refold subtilisin BPN' when added in trans. Here, we constructed chimeric genes with subtilisin E and aqualysin I to attempt the in cis folding of subtilisin E by means of the propeptide of aqualysin I. Our results indicate that the propeptide of aqualysin I can to some extent chaperone the intramolecular folding of the denatured subtilisin E. These results suggest that propeptides in the subtilisin family, despite their sequence diversity, have similar functions. Further, some enzymatic properties of some chimeras in which the subtilisin mature domain is partly swapped with that of aqualysin I were shown to be more similar to those of aqualysin I.

Ubiquitin-associated (UBA) domains in Rad23 bind ubiquitin and promote inhibition of multi-ubiquitin chain assembly

Rad23 is a DNA repair protein that promotes the assembly of the nucleotide excision repair complex. Rad23 can interact with the 26S proteasome through an N-terminal ubiquitin-like domain, and inhibits the assembly of substrate-linked multi-ubiquitin (multi-Ub) chains in vitro and in vivo. Significantly, Rad23 can bind a proteolytic substrate that is conjugated to a few ubiquitin (Ub) moieties. We report here that two ubiquitin-associated (UBA) domains in Rad23 form non-covalent interactions with Ub. A mutant that lacked either UBA sequence was capable of blocking the assembly of substrate-linked multi-Ub chains, although a mutant that lacked both UBA domains was significantly impaired. These studies suggest that the interaction with Ub is required for Rad23 activity, and that other UBA-containing proteins may have a similar function.

Substrate-induced activation of a trapped IMC-mediated protein folding intermediate

While several unfolded proteins acquire native structures through distinct folding intermediates, the physiological relevance and importance of such states in the folding kinetics remain controversial. The intramolecular chaperone (IMC) of subtilisin was used to trap a partially folded, stable crosslinked intermediate conformer (CLIC) through a disulfide bond between mutated IMC and subtilisin. The trapped CLIC contains non-native interactions. Here we show that CLIC can be induced into a catalytically active form by incubating it with small peptide substrates. The structure and catalytic properties of the activated crosslinked intermediate conformer (A-CLIC) differ from those of the fully folded enzyme in that A-CLIC lacks any endopeptidase activity toward a large protein substrate. Our results show that a disulfide-linked partially folded protein can be induced to acquire catalytic activity with a substrate specificity that is different from completely folded subtilisin. These results also suggest that protein folding intermediates may also participate in catalytic reactions.

Folding pathway mediated by an intramolecular chaperone: the structural and functional characterization of the aqualysin I propeptide

Aqualysin I, a thermostable homologue of subtilisin, requires its propeptide (ProA) to function as an intramolecular chaperone (IMC). To decipher the mechanisms through which propeptides can initiate protein folding, we characterized ProA in terms of its sequence, structure and function. Our results show that, in contrast to ProS (propeptide of subtilisin), ProA can fold spontaneously, reversibly and cooperatively into a stable monomeric alpha-beta conformation, even when isolated from its cognate protease-domain. ProA displays an indiscernible amount of tertiary structure with a considerable solvent-accessible hydrophobic surface, but is not a classical molten-globule folding intermediate. Moreover, despite showing only 21 % sequence identity with ProS, ProA can not only inhibit enzymatic activity with a magnitude tenfold greater than ProS, but can also chaperone subtilisin folding, albeit with a lower efficiency. The structure of ProA complexed with subtilisin is different from that of isolated ProA. Hence, additional interactions seem necessary to induce ProA into a compact structure. Our results also suggest that: (a) propeptides that are potent inhibitors are not necessarily better IMCs; (b) propeptides within the subtilase family appear polymorphic and; (c) the intrinsic instability within propeptides may be necessary for rapid activation of the cognate protein.

Folding pathway mediated by an intramolecular chaperone. The inhibitory and chaperone functions of the subtilisin propeptide are not obligatorily linked

The subtilisin propeptide functions as an intramolecular chaperone (IMC) that facilitates correct folding of the catalytic domain while acting like a competitive inhibitor of proteolytic activity. Upon completion of folding, subtilisin initiates IMC degradation to complete precursor maturation. Existing data suggest that the chaperone and inhibitory functions of the subtilisin IMC domain are interdependent during folding. Based on x-ray structure of the IMC-subtilisin complex, we introduce a point mutation (E112A) to disrupt three hydrogen bonds that stabilize the interface between the protease and its IMC domain. This mutation within subtilisin does not alter the folding kinetics but dramatically slows down autoprocessing of the IMC domain. Inhibition of E112A-subtilisin activity by the IMC added in trans is 35-fold weaker than wild-type subtilisin. Although the IMC domain displays substantial loss of inhibitory function, its ability to chaperone E112A-subtilisin folding remains intact. Our results show that (i) the chaperone activity of the IMC domain is not obligatorily linked with its ability to bind with and inhibit active subtilisin; (ii) degradation and not autoprocessing of the IMC domain is the rate-limiting step in precursor maturation; and (iii) the Glu(112) residue within the IMC-subtilisin interface is not crucial for initiating folding but is important in maintaining the IMC structure capable of binding subtilisin.

Intramolecular chaperones: polypeptide extensions that modulate protein folding

Several prokaryotic and eukaryotic proteins are synthesized as precursors in the form of pre-pro-proteins. While the pre-regions function as signal peptides that are involved in transport, the propeptides can often catalyze correct folding of their associated proteins. Such propeptides have been termed intramolecular chaperones. In cases where propeptides may not directly catalyze the folding reaction, it appears that they can facilitate processes such as structural organization and oligomerization, localization, sorting and modulation of enzymatic activity and stability of proteins. Based on the available literature it appears that propeptides may actually function as 'post-translational modulators' of protein structure and function. Propeptides can be classified into two broad categories: Class I propeptides that function as intramolecular chaperones and directly catalyze the folding reaction; and Class II propeptides that are not directly involved in folding.

A pathway for conformational diversity in proteins mediated by intramolecular chaperones

Conformational diversity within unique amino acid sequences is observed in diseases like scrapie and Alzheimer's disease. The molecular basis of such diversity is unknown. Similar phenomena occur in subtilisin, a serine protease homologous with eukaryotic pro-hormone convertases. The subtilisin propeptide functions as an intramolecular chaperone (IMC) that imparts steric information during folding but is not required for enzymatic activity. Point mutations within IMCs alter folding, resulting in structural conformers that specifically interact with their cognate IMCs in a process termed "protein memory." Here, we show a mechanism that mediates conformational diversity in subtilisin. During maturation, while the IMC is autocleaved and subsequently degraded by the active site of subtilisin, enzymatic properties of this site differ significantly before and after cleavage. Although subtilisin folded by Ile-48 --> Thr IMC (IMCI-48T) acquires an "altered" enzymatically active conformation (SubI-48T) significantly different from wild-type subtilisin (SubWT), both precursors undergo autocleavage at similar rates. IMC cleavage initiates conformational changes during which the IMC continues its chaperoning function subsequent to its cleavage from subtilisin. Structural imprinting resulting in conformational diversity originates during this reorganization stage and is a late folding event catalyzed by autocleavage of the IMC.

The crystal structure of an autoprocessed Ser221Cys-subtilisin E-propeptide complex at 2.0 A resolution

We report here the crystallographic structure determination of an autoprocessed (Ser221Cys)-subtilisin E-propeptide complex at 2.0 A resolution. The subtilisin domain sequence has a single substitution (Ser221Cys) which has been shown to block the maturation process prior to degradation of the propeptide domain (77 residues) that acts as an intramolecular chaperon. This mutation, however, did not prevent the enzyme from cleaving its propeptide domain with a 60-80% efficiency. The current determination is the first example of a subtilisin E-propeptide complex which has been autoprocessed. A previous structure determination of a BPN'-prosegment complex has been reported in which the subtilisin domain was extensively mutated and a calcium binding loop was deleted. Further, in this earlier determination, the complex was formed by the addition of separately expressed propeptide domain. The structure determination reported here provides additional information about the nature of the interaction between the subtilisin and propeptide domains in this complex.

Folding pathway mediated by an intramolecular chaperone: characterization of the structural changes in pro-subtilisin E coincident with autoprocessing

Mechanisms by which many N-terminal propeptides facilitate folding of proteins are unknown. The maturation of such proteins from their precursors involve three steps, namely: (1) folding of the precursor, (2) autoprocessing of the propeptide from the N terminus and (3) degradation of the cleaved propeptide. Using subtilisin E we have analyzed the mechanism of propeptide-mediated protein folding. Two active site mutations allow us to trap intermediates at stages of autoprocessing and degradation. An analysis of these intermediates has shown the existence of a molten-globule-like intermediate on the folding pathway. After autoprocessing of the propeptide, this intermediate undergoes a structural reorganization which reduces solvent-accessible hydrophobic surface area and increases the amount of its tertiary structure. Removal of the propeptide from the mature enzyme in this intermediate state occurs only by proteolytic degradation and contributes to the stability of the active enzyme.

Folding mediated by an intramolecular chaperone: autoprocessing pathway of the precursor resolved via a substrate assisted catalysis mechanism

Subtilisin is synthesized with an N-terminal propeptide which has been demonstrated to function as an intramolecular chaperone that is only essential for the folding of the active enzyme. After folding, the propeptide is removed via an intramolecular autoprocessing mechanism. This mechanism is blocked when His64, a member of the catalytic triad is substituted with Ala. However, an additional mutation in the propeptide substituting Glu-2 with His was able to suppress the His64Ala mutation, allowing autoprocessing of the propeptide. This suppression is considered to be due to a "substrate assisted catalysis" mechanism and demonstrates that the cleavage to the subtilisin propeptide is an autocatalytic process.

The structural and functional organization of intramolecular chaperones: the N-terminal propeptides which mediate protein folding

A large number of prokaryotic as well as eukaryotic proteins are produced with amino terminal propeptides. These amino-terminal extensions are essential for mediating proper folding of their corresponding proteins and are also termed as intramolecular chaperones. Though these propeptides are highly specific and unique in their function, several common features have been identified and indicate that the overall mechanism by which they function may be very similar.

Intramolecular chaperones and protein folding

Many proteins from both prokaryotic and eukaryotic sources are produced with amino-terminal propeptides. These propeptides, which are usually located between the signal peptide and the mature protein, are essential for the proper function of that protein. Recent research has indicated that these polypeptides are indispensible for proper folding of the proteins they are attached to. As propeptides perform a function similar to that of a large family of heat shock proteins, they had been broadly classified as molecular chaperones. However, significant differences exist between these two classes of proteins and to distinguish them from one another, propeptides have been termed intramolecular chaperones. Recent results have suggested that such intramolecular chaperones may be found in a large number of proteins.

Folding pathway mediated by an intramolecular chaperone

The N-terminal propeptide of subtilisin, a serine protease, functions as an intramolecular chaperone which is crucial for proper folding of the active enzyme. This nascent N-terminal propeptide is removed after completion of the folding process. Here we present a possible pathway by which intramolecular chaperones mediate protein folding. Using circular dichroism to analyze acid-denatured subtilisin we have identified a folding-competent state which can refold to an active conformation in the absence of the propeptide. Earlier work had shown that guanidine hydrochloride-denatured subtilisin was in a state incapable of folding in absence of its propeptide. Comparison of the folding-incompetent and folding-competent states indicates that refolding is facilitated by the presence of residual structure present only in the folding-competent state. The analysis further indicates that the propeptide is essential for inducing this state. Therefore the folding-competent state may lie on--or be in rapid equilibrium with an intermediate on--the folding pathway of subtilisin. In the absence of the propeptide, formation of such a state--and hence refolding--is extremely slow.