Characterization of a novel acylaminoacyl peptidase with hexameric structure and endopeptidase activity

Crystal structure and solution scattering of Geobacillus stearothermophilus S9 peptidase reveal structural adaptations for carboxypeptidase activity

Acylaminoacyl peptidases (AAPs) play a pivotal role in various pathological conditions and are recognized as potential therapeutic targets. AAPs exhibit a wide range of activities, such as acylated amino acid-dependent aminopeptidase, endopeptidase, and less studied carboxypeptidase activity. We have determined the crystal structure of an AAP from Geobacillus stearothermophilus (S9gs) at 2.0 Å resolution. Despite being annotated as an aminopeptidase in the NCBI database, our enzymatic characterization proved S9gs to be a carboxypeptidase. Solution-scattering studies showed that S9gs exists as a tetramer in solution, and crystal structure analysis revealed adaptations responsible for the carboxypeptidase activity of S9gs. The findings present a hypothesis for substrate selection, substrate entry, and product exit from the active site, enriching our understanding of this rare carboxypeptidase.

A deeply conserved protease, acylamino acid-releasing enzyme (AARE), acts in ageing in Physcomitrella and Arabidopsis

Reactive oxygen species (ROS) are constant by-products of aerobic life. In excess, ROS lead to cytotoxic protein aggregates, which are a hallmark of ageing in animals and linked to age-related pathologies in humans. Acylamino acid-releasing enzymes (AARE) are bifunctional serine proteases, acting on oxidized proteins. AARE are found in all domains of life, albeit under different names, such as acylpeptide hydrolase (APEH/ACPH), acylaminoacyl peptidase (AAP), or oxidized protein hydrolase (OPH). In humans, AARE malfunction is associated with age-related pathologies, while their function in plants is less clear. Here, we provide a detailed analysis of AARE genes in the plant lineage and an in-depth analysis of AARE localization and function in the moss Physcomitrella and the angiosperm Arabidopsis. AARE loss-of-function mutants have not been described for any organism so far. We generated and analysed such mutants and describe a connection between AARE function, aggregation of oxidized proteins and plant ageing, including accelerated developmental progression and reduced life span. Our findings complement similar findings in animals and humans, and suggest a unified concept of ageing may exist in different life forms.

Cryo-EM structure of acylpeptide hydrolase reveals substrate selection by multimerization and a multi-state serine-protease triad

The first structure of tetrameric mammalian acylaminoacyl peptidase, an enzyme that functions as an upstream regulator of the proteasome through the removal of terminal N-acetylated residues from its protein substrates, was determined by cryo-EM and further elucidated by MD simulations. Self-association results in a toroid-shaped quaternary structure, guided by an amyloidogenic β-edge and unique inserts. With a Pro introduced into its central β-sheet, sufficient conformational freedom is awarded to the segment containing the catalytic Ser587 that the serine protease catalytic triad alternates between active and latent states. Active site flexibility suggests that the dual function of catalysis and substrate selection are fulfilled by a novel mechanism: substrate entrance is regulated by flexible loops creating a double-gated channel system, while binding of the substrate to the active site is required for stabilization of the catalytic apparatus – as a second filter before hydrolysis. The structure not only underlines that within the family of S9 proteases homo-multimerization acts as a crucial tool for substrate selection, but it will also allow drug design targeting of the ubiquitin-proteasome system.

The structure of tetrameric mammalian acylaminoacyl peptidase – a key upstream regulator of the proteasome – was determined by cryo-EM (and elucidated by MD), showing a “shutters-and-channels” substrate selection apparatus created by oligomerization.

Achieving Functionality Through Modular Build-up: Structure and Size Selection of Serine Oligopeptidases

Enzymes of the prolyl oligopeptidase family (S9 family) recognize their substrates not only by the specificity motif to be cleaved but also by size - they hydrolyze oligopeptides smaller than 30 amino acids. They belong to the serine-protease family, but differ from classical serine-proteases in size (80 kDa), structure (two domains) and regulation system (size selection of substrates). This group of enzymes is an important target for drug design as they are linked to amnesia, schizophrenia, type 2 diabetes, trypanosomiasis, periodontitis and cell growth. By comparing the structure of various members of the family we show that the most important features contributing to selectivity and efficiency are: (i) whether the interactions weaving the two domains together play a role in stabilizing the catalytic triad and thus their absence may provide for its deactivation: these oligopeptidases can screen their substrates by opening up, and (ii) whether the interaction-prone β-edge of the hydrolase domain is accessible and thus can guide a multimerization process that creates shielded entrance or intricate inner channels for the size-based selection of substrates. These cornerstones can be used to estimate the multimeric state and selection strategy of yet undetermined structures.

Proteolytic systems of archaea: slicing, dicing, and mincing in the extreme

Archaea are phylogenetically distinct from bacteria, and some of their proteolytic systems reflect this distinction. Here, the current knowledge of archaeal proteolysis is reviewed as it relates to protein metabolism, protein homeostasis, and cellular regulation including targeted proteolysis by proteasomes associated with AAA-ATPase networks and ubiquitin-like modification. Proteases and peptidases that facilitate the recycling of peptides to amino acids as well as membrane-associated and integral membrane proteases are also reviewed.

Catalytically distinct states captured in a crystal lattice: the substrate-bound and scavenger states of acylaminoacyl peptidase and their implications for functionality

Acylaminoacyl peptidase (AAP) is an oligopeptidase that only cleaves short peptides or protein segments. In the case of AAP from Aeropyrum pernix (ApAAP), previous studies have led to a model in which the clamshell-like opening and closing of the enzyme provides the means of substrate-size selection. The closed form of the enzyme is catalytically active, while opening deactivates the catalytic triad. The crystallographic results presented here show that the open form of ApAAP is indeed functionally disabled. The obtained crystal structures also reveal that the closed form is penetrable to small ligands: inhibitor added to the pre-formed crystal was able to reach the active site of the rigidified protein, which is only possible through the narrow channel of the propeller domain. Molecular-dynamics simulations investigating the structure of the complexes formed with longer peptide substrates showed that their binding within the large crevice of the closed form of ApAAP leaves the enzyme structure unperturbed; however, their accessing the binding site seems more probable when assisted by opening of the enzyme. Thus, the open form of ApAAP corresponds to a scavenger of possible substrates, the actual cleavage of which only takes place if the enzyme is able to re-close.

A self-compartmentalizing hexamer serine protease from Pyrococcus horikoshii: substrate selection achieved through multimerization

Oligopeptidases impose a size limitation on their substrates, the mechanism of which has long been under debate. Here we present the structure of a hexameric serine protease, an oligopeptidase from Pyrococcus horikoshii (PhAAP), revealing a complex, self-compartmentalized inner space, where substrates may access the monomer active sites passing through a double-gated "check-in" system, first passing through a pore on the hexamer surface and then turning to enter through an even smaller opening at the monomers' domain interface. This substrate screening strategy is unique within the family. We found that among oligopeptidases, a residue of the catalytic apparatus is positioned near an amylogenic β-edge, which needs to be protected to prevent aggregation, and we found that different oligopeptidases use different strategies to achieve such an end. We propose that self-assembly within the family results in characteristically different substrate selection mechanisms coupled to different multimerization states.

Reciprocal influence of protein domains in the cold-adapted acyl aminoacyl peptidase from Sporosarcina psychrophila

Acyl aminoacyl peptidases are two-domain proteins composed by a C-terminal catalytic α/β-hydrolase domain and by an N-terminal β-propeller domain connected through a structural element that is at the N-terminus in sequence but participates in the 3D structure of the C-domain. We investigated about the structural and functional interplay between the two domains and the bridge structure (in this case a single helix named α1-helix) in the cold-adapted enzyme from Sporosarcina psychrophila (SpAAP) using both protein variants in which entire domains were deleted and proteins carrying substitutions in the α1-helix. We found that in this enzyme the inter-domain connection dramatically affects the stability of both the whole enzyme and the β-propeller. The α1-helix is required for the stability of the intact protein, as in other enzymes of the same family; however in this psychrophilic enzyme only, it destabilizes the isolated β-propeller. A single charged residue (E10) in the α1-helix plays a major role for the stability of the whole structure. Overall, a strict interaction of the SpAAP domains seems to be mandatory for the preservation of their reciprocal structural integrity and may witness their co-evolution.

Coupled motions during dynamics reveal a tunnel toward the active site regulated by the N-terminal α-helix in an acylaminoacyl peptidase

Acylaminoacyl peptidase (AAP) subfamily belongs to the prolyl oligopeptidase (POP) family of serine-proteases. There is a great interest in the definition of molecular mechanisms related to the activity and substrate recognition of these complex multi-domain enzymes. The active site relies at the interface between the C-terminal catalytic domain and the β-propeller domain, whose N-terminal region acts as a bridge to the hydrolase domain. In AAP, the N-terminal extension is characterized by a structurally conserved α1-helix, which is known to affect thermal stability and thermal dependence of the catalytic activity. In the present contribution, results from hundreds nanosecond all-atom molecular dynamics simulations, along with analyses of the networks of cross-correlated motions of a member of the AAP subfamily are discussed. The MD investigation identifies a tunnel that from the surrounding of the N-terminal α1-helix bring to the catalytic site. This cavity seems to be regulated by conformational changes of the α1-helix itself during the dynamics. The evidence here provided can be a useful guide for a better understanding of the mechanistic aspects related to AAP activity, but also for drug design purposes.

Identification and characterisation of a novel acylpeptide hydrolase from Sulfolobus solfataricus: structural and functional insights

A novel acylpeptide hydrolase, named APEH-3_Ss, was isolated from the hypertermophilic archaeon Sulfolobus solfataricus. APEH is a member of the prolyl oligopeptidase family which catalyzes the removal of acetylated amino acid residues from the N terminus of oligopeptides. The purified enzyme shows a homotrimeric structure, unique among the associate partners of the APEH cluster and, in contrast to the archaeal APEHs which show both exo/endo peptidase activities, it appears to be a “true” aminopeptidase as exemplified by its mammalian counterparts, with which it shares a similar substrate specificity. Furthermore, a comparative study on the regulation of apeh gene expression, revealed a significant but divergent alteration in the expression pattern of apeh-3_Ss and apeh_Ss (the gene encoding the previously identified APEH_Ss from S. solfataricus), which is induced in response to various stressful growth conditions. Hence, both APEH enzymes can be defined as stress-regulated proteins which play a complementary role in enabling the survival of S. solfataricus cells under different conditions. These results provide new structural and functional insights into S. solfataricus APEH, offering a possible explanation for the multiplicity of this enzyme in Archaea.