A Lon-like protease with no ATP-powered unfolding activity

Unique Structural Fold of LonBA Protease from Bacillus subtilis, a Member of a Newly Identified Subfamily of Lon Proteases

ATP-dependent Lon proteases are key participants in the quality control system that supports the homeostasis of the cellular proteome. Based on their unique structural and biochemical properties, Lon proteases have been assigned in the MEROPS database to three subfamilies (A, B, and C). All Lons are single-chain, multidomain proteins containing an ATPase and protease domains, with different additional elements present in each subfamily. LonA and LonC proteases are soluble cytoplasmic enzymes, whereas LonBs are membrane-bound. Based on an analysis of the available sequences of Lon proteases, we identified a number of enzymes currently assigned to the LonB subfamily that, although presumably membrane-bound, include structural features more similar to their counterparts in the LonA subfamily. This observation was confirmed by the crystal structure of the proteolytic domain of the enzyme previously assigned as Bacillus subtilis LonB, combined with the modeled structure of its ATPase domain. Several structural features present in both domains differ from their counterparts in either LonA or LonB subfamilies. We thus postulate that this enzyme is the founding member of a newly identified LonBA subfamily, so far found only in the gene sequences of firmicutes.

Structure and the mode of activity of Lon proteases from diverse organisms

Lon proteases, members of the AAA⁺ superfamily of enzymes, are key components of the protein quality control system in bacterial cells, as well as in the mitochondria and other specialized organelles of higher organisms. These enzymes have been subject of extensive biochemical and structural investigations, resulting in 72 crystal and solution structures, including structures of the individual domains, multi-domain constructs, and full-length proteins. However, interpretation of the latter structures still leaves some questions unanswered. Based on their amino acid sequence and details of their structure, Lon proteases can be divided into at least three subfamilies, designated as LonA, LonB, and LonC. Protomers of all Lons are single-chain polypeptides and contain two functional domains, ATPase and protease. The LonA enzymes additionally include a large N-terminal region, and different Lons may also include non-conserved inserts in the principal domains. These ATP-dependent proteases function as homohexamers, in which unfolded substrates are translocated to a large central chamber where they undergo proteolysis by a processive mechanism. X-ray crystal structures provided high-resolution models which verified that Lons are hydrolases with the rare Ser-Lys catalytic dyad. Full-length LonA enzymes have been investigated by cryo-electron microscopy (cryo-EM), providing description of the functional enzyme at different stages of the catalytic cycle, indicating extensive flexibility of their N-terminal domains, and revealing insights into the substrate translocation mechanism. Structural studies of Lon proteases provide an interesting case for symbiosis of X-ray crystallography and cryo-EM, currently the two principal techniques for determination of macromolecular structures.

Processive cleavage of substrate at individual proteolytic active sites of the Lon protease complex

The Lon protease is the prototype of a family of proteolytic machines with adenosine triphosphatase modules built into a substrate degradation chamber. Lon is known to degrade protein substrates in a processive fashion, cutting a protein chain processively into small peptides before commencing cleavages of another protein chain. Here, we present structural and biochemical evidence demonstrating that processive substrate degradation occurs at each of the six proteolytic active sites of Lon, which forms a deep groove that partially encloses the substrate polypeptide chain by accommodating only the unprimed residues and permits processive cleavage in the C-to-N direction. We identify a universally conserved acidic residue at the exit side of the binding groove indispensable for the proteolytic activity. This noncatalytic residue likely promotes processive proteolysis by carboxyl-carboxylate interactions with cleaved intermediates. Together, these results uncover a previously unrecognized mechanism for processive substrate degradation by the Lon protease.

Molecular basis for ATPase-powered substrate translocation by the Lon AAA+ protease

The Lon AAA+ (adenosine triphosphatases associated with diverse cellular activities) protease (LonA) converts ATP-fuelled conformational changes into sufficient mechanical force to drive translocation of a substrate into a hexameric proteolytic chamber. To understand the structural basis for the substrate translocation process, we determined the cryo-electron microscopy (cryo-EM) structure of Meiothermus taiwanensis LonA (MtaLonA) in a substrate-engaged state at 3.6 Å resolution. Our data indicate that substrate interactions are mediated by the dual pore loops of the ATPase domains, organized in spiral staircase arrangement from four consecutive protomers in different ATP-binding and hydrolysis states. However, a closed AAA+ ring is maintained by two disengaged ADP-bound protomers transiting between the lowest and highest position. This structure reveals a processive rotary translocation mechanism mediated by LonA-specific nucleotide-dependent allosteric coordination among the ATPase domains, which is induced by substrate binding.

Structural Basis for the Magnesium-Dependent Activation and Hexamerization of the Lon AAA+ Protease

The Lon AAA+ protease (LonA) plays important roles in protein homeostasis and regulation of diverse biological processes. LonA behaves as a homomeric hexamer in the presence of magnesium (Mg(2+)) and performs ATP-dependent proteolysis. However, it is also found that LonA can carry out Mg(2+)-dependent degradation of unfolded protein substrate in an ATP-independent manner. Here we show that in the presence of Mg(2+) LonA forms a non-secluded hexameric barrel with prominent openings, which explains why Mg(2+)-activated LonA can operate as a diffusion-based chambered protease to degrade unstructured protein and peptide substrates efficiently in the absence of ATP. A 1.85 Å crystal structure of Mg(2+)-activated protease domain reveals Mg(2+)-dependent remodeling of a substrate-binding loop and a potential metal-binding site near the Ser-Lys catalytic dyad, supported by biophysical binding assays and molecular dynamics simulations. Together, these findings reveal the specific roles of Mg(2+) in the molecular assembly and activation of LonA.

The Lon protease homologue LonA, not LonC, contributes to the stress tolerance and biofilm formation of Actinobacillus pleuropneumoniae

Lon proteases are a family of ATP-dependent proteases that are involved in the degradation of abnormal proteins in bacteria exposed to adverse environmental stress. An analysis of the genome sequence of Actinobacillus pleuropneumoniae revealed the unusual presence of two putative ATP-dependent Lon homologues, LonA and LonC. Sequence comparisons indicated that LonA has the classical domain organization of the LonA subfamily, which includes the N-terminal domain, central ATPase (AAA) domain, and C-terminal proteolytic (P) domain. LonC belongs to the recently classified LonC subfamily, which includes Lon proteases that contain neither the N-terminal domain of LonA nor the transmembrane region that is present only in LonB subfamily members. To investigate the roles of LonA and LonC in A. pleuropneumoniae, mutants with deletions in the lonA and lonC genes were constructed. The impaired growth of the △lonA mutant exposed to low and high temperatures and osmotic and oxidative stress conditions indicates that the LonA protease is required for the stress tolerance of A. pleuropneumoniae. Furthermore, the △lonA mutant exhibited significantly reduced biofilm formation compared to the wild-type strain. However, no significant differences in stress responses or biofilm formation were observed between the △lonC mutant and the wild-type strain. The △lonA mutant exhibited reduced colonization ability and attenuated virulence of A. pleuropneumoniae in the BALB/c mouse model compared to the wild-type strain. Disruption of lonC gene did not significantly influence the colonization and virulence of A. pleuropneumoniae. The data presented in this study illustrate that the LonA protease, but not the LonC protease, is required for the stress tolerance, biofilm formation and pathogenicity of A. pleuropneumoniae.

Structural basis for the ATP-independent proteolytic activity of LonB proteases and reclassification of their AAA+ modules

Lon proteases degrade defective or denature proteins as well as some folded proteins for the control of cellular protein quality. There are two types of Lon proteases, LonA and LonB. Each consists of two functional components: a protease component and an ATPase associated with various cellular activities (AAA+ module). Here, we report the 2.03 -resolution crystal structure of the isolated AAA+ module (iAAA+ module) of LonB from Thermococcus onnurineus NA1 (TonLonB). The iAAA+ module, having no bound nucleotide, adopts a conformation virtually identical to the ADP-bound conformation of AAA+ modules in the hexameric structure of TonLonB; this provides insights into the ATP-independent proteolytic activity observed in a LonB protease. Structural comparison of AAA+ modules between LonA and LonB revealed that the AAA+ modules of Lon proteases are separated into two distinct clades depending on their structural features. The AAA+ module of LonB belongs to the -H2 & Ins1 insert clade (HINS clade)- defined for the first time in this study, while the AAA+ module of LonA is a member of the HCLR clade.

The N-terminal substrate-recognition domain of a LonC protease exhibits structural and functional similarity to cytosolic chaperones

The Lon protease is ubiquitous in nature. Its proteolytic activity is associated with diverse cellular functions ranging from maintaining proteostasis under normal and stress conditions to regulating cell metabolism. Although Lon was originally identified as an ATP-dependent protease with fused AAA+ (ATPases associated with diverse cellular activities) and protease domains, analyses have recently identified LonC as a class of Lon-like proteases with no intrinsic ATPase activity. In contrast to the canonical ATP-dependent Lon present in eukaryotic organelles and prokaryotes, LonC contains an AAA-like domain that lacks the conserved ATPase motifs. Moreover, the LonC AAA-like domain is inserted with a large domain predicted to be largely α-helical; intriguingly, this unique Lon-insertion domain (LID) was disordered in the recently determined full-length crystal structure of Meiothermus taiwanensis LonC (MtaLonC). Here, the crystal structure of the N-terminal AAA-like α/β subdomain of MtaLonC containing an intact LID, which forms a large α-helical hairpin protruding from the AAA-like domain, is reported. The structure of the LID is remarkably similar to the tentacle-like prong of the periplasmic chaperone Skp. It is shown that the LID of LonC is involved both in Skp-like chaperone activity and in recognition of unfolded protein substrates. The structure allows the construction of a complete model of LonC with six helical hairpin extensions defining a basket-like structure atop the AAA ring and encircling the entry portal to the barrel-like degradation chamber of Lon.

Structures of an ATP-independent Lon-like protease and its complexes with covalent inhibitors

The Lon proteases are a unique family of chambered proteases with a built-in AAA+ (ATPases associated with diverse cellular activities) module. Here, crystal structures of a unique member of the Lon family with no intrinsic ATPase activity in the proteolytically active form are reported both alone and in complexes with three covalent inhibitors: two peptidomimetics and one derived from a natural product. This work reveals the unique architectural features of an ATP-independent Lon that selectively degrades unfolded protein substrates. Importantly, these results provide mechanistic insights into the recognition of inhibitors and polypeptide substrates within the conserved proteolytic chamber, which may aid the development of specific Lon-protease inhibitors.