Evidence for the existence of elaborate enzyme complexes in the Paleoarchean era.

Due to the lack of macromolecular fossils, the enzymatic repertoire of extinct species has remained largely unknown to date. In an attempt to solve this problem, we have characterized a cyclase subunit (HisF) of the imidazole glycerol phosphate synthase (ImGP-S), which was reconstructed from the era of the last universal common ancestor of cellular organisms (LUCA). As observed for contemporary HisF proteins, the crystal structure of LUCA-HisF adopts the (ßα)8-barrel architecture, one of the most ancient folds. Moreover, LUCA-HisF (i) resembles extant HisF proteins with regard to internal 2-fold symmetry, active site residues, and a stabilizing salt bridge cluster, (ii) is thermostable and shows a folding mechanism similar to that of contemporary (ßα)8-barrel enzymes, (iii) displays high catalytic activity, and (iv) forms a stable and functional complex with the glutaminase subunit (HisH) of an extant ImGP-S. Furthermore, we show that LUCA-HisF binds to a reconstructed LUCA-HisH protein with high affinity. Our findings suggest that the evolution of highly efficient enzymes and enzyme complexes has already been completed in the LUCA era, which means that sophisticated catalytic concepts such as substrate channeling and allosteric communication existed already 3.5 billion years ago.

Folding mechanism of an extremely thermostable (ßα)(8)-barrel enzyme: a high kinetic barrier protects the protein from denaturation.

HisF, the cyclase subunit of imidazole glycerol phosphate synthase (ImGPS) from Thermotoga maritima, is an extremely thermostable (ßα)(8)-barrel protein. We elucidated the unfolding and refolding mechanism of HisF. Its unfolding transition is reversible and adequately described by the two-state model, but 6 weeks is necessary to reach equilibrium (at 25 °C). During refolding, initially a burst-phase off-pathway intermediate is formed. The subsequent productive folding occurs in two kinetic phases with time constants of ~3 and ~20 s. They reflect a sequential process via an on-pathway intermediate, as revealed by stopped-flow double-mixing experiments. The final step leads to native HisF, which associates with the glutaminase subunit HisH to form the functional ImGPS complex. The conversion of the on-pathway intermediate to the native protein results in a 10(6)-fold increase of the time constant for unfolding from 89 ms to 35 h (at 4.0 M GdmCl) and thus establishes a high energy barrier to denaturation. We conclude that the extra stability of HisF is used for kinetic protection against unfolding. In its refolding mechanism, HisF resembles other (ßα)(8)-barrel proteins.

Conservation of the folding mechanism between designed primordial (ßα)8-barrel proteins and their modern descendant.

The (ßα)(8)-barrel is among the most ancient, frequent, and versatile enzyme structures. It was proposed that modern (ßα)(8)-barrel proteins have evolved from an ancestral (ßα)(4)-half-barrel by gene duplication and fusion. We explored whether the mechanism of protein folding has remained conserved during this long-lasting evolutionary process. For this purpose, potential primordial (ßα)(8)-barrel proteins were constructed by the duplication of a (ßα)(4) element of a modern (ßα)(8)-barrel protein, imidazole glycerol phosphate synthase (HisF), followed by the optimization of the initial construct. The symmetric variant Sym1 was less stable than HisF and its crystal structure showed disorder in the contact regions between the half-barrels. The next generation variant Sym2 was more stable than HisF, and the contact regions were well resolved. Remarkably, both artificial (ßα)(8)-barrels show the same refolding mechanism as HisF and other modern (ßα)(8)-barrel proteins. Early in folding, they all equilibrate rapidly with an off-pathway species. On the productive folding path, they form closely related intermediates and reach the folded state with almost identical rates. The high energy barrier that synchronizes folding is thus conserved. The strong differences in stability between these proteins develop only after this barrier and lead to major changes in the unfolding rates. We conclude that the refolding mechanism of (ßα)(8)-barrel proteins is robust. It evolved early and, apparently, has remained conserved upon the diversification of sequences and functions that have taken place within this large protein family.

Computational and experimental evidence for the evolution of a (beta alpha)8-barrel protein from an ancestral quarter-barrel stabilised by disulfide bonds.

The evolution of the prototypical (beta alpha)(8)-barrel protein imidazole glycerol phosphate synthase (HisF) was studied by complementary computational and experimental approaches. The 4-fold symmetry of HisF suggested that its constituting (beta alpha)(2) quarter-barrels have a common evolutionary origin. This conclusion was supported by the computational reconstruction of the HisF sequence of the last common ancestor, which showed that its quarter-barrels were more similar to each other than are those of extant HisF proteins. A comprehensive sequence analysis identified HisF-N1 [corresponding to (beta alpha)(1-2)] as the slowest evolving quarter-barrel. This finding indicated that it is the closest relative of the common (beta alpha)(2) predecessor, which must have been a stable and presumably tetrameric protein. In accordance with this prediction, a recombinantly produced HisF-N1 protein was properly folded and formed a tetramer being stabilised by disulfide bonds. The introduction of a disulfide bond in HisF-C1 [corresponding to (beta alpha)(5-6)] also resulted in the formation of a stable tetramer. The fusion of two identical HisF-N1 quarter-barrels yielded the stable dimeric half-barrel HisF-N1N1. Our findings suggest a two-step evolutionary pathway in which a HisF-N1-like predecessor was duplicated and fused twice to yield HisF. Most likely, the (beta alpha)(2) quarter-barrel and (beta alpha)(4) half-barrel intermediates on this pathway were stabilised by disulfide bonds that became dispensable upon consolidation of the (beta alpha)(8)-barrel.

Consequences of domain insertion on the stability and folding mechanism of a protein.

SlyD, the sensitive-to-lysis protein from Escherichia coli, consists of two domains. They are not arranged successively along the protein chain, but one domain, the "insert-in-flap" (IF) domain, is inserted internally as a guest into a surface loop of the host domain, which is a prolyl isomerase of the FK506 binding protein (FKBP) type. We used SlyD as a model to elucidate how such a domain insertion affects the stability and folding mechanism of the host and the guest domain. For these studies, the two-domain protein was compared with a single-domain variant SlyDDeltaIF, SlyD* without the chaperone domain (residues 1-69 and 130-165) in which the IF domain was removed and replaced by a short loop, as present in human FKBP12. Equilibrium unfolding and folding kinetics followed an apparent two-state mechanism in the absence and in the presence of the IF domain. The inserted domain decreased, however, the stability of the host domain in the transition region and decelerated its refolding reaction by about 10-fold. This originates from the interruption of the chain connectivity by the IF domain and its inherent instability. To monitor folding processes in this domain selectively, a Trp residue was introduced as fluorescent probe. Kinetic double-mixing experiments revealed that, in intact SlyD, the IF domain folds and unfolds about 1000-fold more rapidly than the FKBP domain, and that it is strongly stabilized when linked with the folded FKBP domain. The unfolding limbs of the kinetic chevrons of SlyD show a strong downward curvature. This deviation from linearity is not caused by a transition-state movement, as often assumed, but by the accumulation of a silent unfolding intermediate at high denaturant concentrations. In this kinetic intermediate, the FKBP domain is still folded, whereas the IF domain is already unfolded.