Communication between ClpX and ClpP during substrate processing and degradation

A camel passes through the eye of a needle: protein unfolding activity of Clp ATPases

Clp ATPases are protein machines involved in protein degradation and disaggregation. The common structural feature of Clp ATPases is the formation of ring-shaped oligomers. Recent work has shown that the function of all Clp ATPases is based on an energy-dependent threading of substrates through the narrow pore at the centre of the ring. This review gives an outline of known mechanistic principles of threading machines that unfold protein substrates either before their degradation (ClpA, ClpX, HslU) or during their reactivation from aggregates (ClpB). The place of Clp ATPases within a broad AAA+ superfamily of ATPases associated with various cellular activities suggests that similar mechanisms can be used by other protein machines to induce conformational rearrangements in a wide variety of substrates.

The molecular architecture of the metalloprotease FtsH

The ATP-dependent integral membrane protease FtsH is universally conserved in bacteria. Orthologs exist in chloroplasts and mitochondria, where in humans the loss of a close FtsH-homolog causes a form of spastic paraplegia. FtsH plays a crucial role in quality control by degrading unneeded or damaged membrane proteins, but it also targets soluble signaling factors like sigma(32) and lambda-CII. We report here the crystal structure of a soluble FtsH construct that is functional in caseinolytic and ATPase assays. The molecular architecture of this hexameric molecule consists of two rings where the protease domains possess an all-helical fold and form a flat hexagon that is covered by a toroid built by the AAA domains. The active site of the protease classifies FtsH as an Asp-zincin, contrary to a previous report. The different symmetries of protease and AAA rings suggest a possible translocation mechanism of the target polypeptide chain into the interior of the molecule where the proteolytic sites are located.

The asymmetry in the mature amino-terminus of ClpP facilitates a local symmetry match in ClpAP and ClpXP complexes

ClpP is a self-compartmentalized proteolytic assembly comprised of two, stacked, heptameric rings that, when associated with its cognate hexameric ATPase (ClpA or ClpX), form the ClpAP and ClpXP ATP-dependent protease, respectively. The symmetry mismatch is an absolute feature of this large energy-dependent protease and also of the proteasome, which shares a similar barrel-shaped architecture, but how it is accommodated within the complex has yet to be understood, despite recent structural investigations, due in part to the conformational lability of the N-termini. We present the structures of Escherichia coli ClpP to 1.9A and an inactive variant that provide some clues for how this might be achieved. In the wild type protein, the highly conserved N-terminal 20 residues can be grouped into two major structural classes. In the first, a loop formed by residues 10-15 protrudes out of the central access channel extending approximately 12-15A from the surface of the oligomer resulting in the closing of the access channel observed in one ring. Similar loops are implied to be exclusively observed in human ClpP and a variant of ClpP from Streptococcus pneumoniae. In the other ring, a second class of loop is visible in the structure of wt ClpP from E. coli that forms closer to residue 16 and faces toward the interior of the molecule creating an open conformation of the access channel. In both classes, residues 18-20 provide a conserved interaction surface. In the inactive variant, a third class of N-terminal conformation is observed, which arises from a conformational change in the position of F17. We have performed a detailed functional analysis on each of the first 20 amino acid residues of ClpP. Residues that extend beyond the plane of the molecule (10-15) have a lesser effect on ATPase interaction than those lining the pore (1-7 and 16-20). Based upon our structure-function analysis, we present a model to explain the widely disparate effects of individual residues on ClpP-ATPase complex formation and also a possible functional reason for this mismatch.

Rebuilt AAA + motors reveal operating principles for ATP-fuelled machines

Hexameric ring-shaped ATPases of the AAA + (for ATPases associated with various cellular activities) superfamily power cellular processes in which macromolecular structures and complexes are dismantled or denatured, but the mechanisms used by these machine-like enzymes are poorly understood. By covalently linking active and inactive subunits of the ATPase ClpX to form hexamers, here we show that diverse geometric arrangements can support the enzymatic unfolding of protein substrates and translocation of the denatured polypeptide into the ClpP peptidase for degradation. These studies indicate that the ClpX power stroke is generated by ATP hydrolysis in a single subunit, rule out concerted and strict sequential ATP hydrolysis models, and provide evidence for a probabilistic sequence of nucleotide hydrolysis. This mechanism would allow any ClpX subunit in contact with a translocating polypeptide to hydrolyse ATP to drive substrate spooling into ClpP, and would prevent stalling if one subunit failed to bind or hydrolyse ATP. Energy-dependent machines with highly diverse quaternary architectures and molecular functions could operate by similar asymmetric mechanisms.

Molecular chaperones in protein quality control

Proteins must fold into their correct three-dimensional conformation in order to attain their biological function. Conversely, protein aggregation and misfolding are primary contributors to many devastating human diseases, such as prion-mediated infections, Alzheimer's disease, type II diabetes and cystic fibrosis. While the native conformation of a polypeptide is encoded within its primary amino acid sequence and is sufficient for protein folding in vitro, the situation in vivo is more complex. Inside the cell, proteins are synthesized or folded continuously; a process that is greatly assisted by molecular chaperones. Molecular chaperones are a group of structurally diverse and mechanistically distinct proteins that either promote folding or prevent the aggregation of other proteins. With our increasing understanding of the proteome, it is becoming clear that the number of proteins that can be classified as molecular chaperones is increasing steadily. Many of these proteins have novel but essential cellular functions that differ from that of more "conventional" chaperones, such as Hsp70 and the GroE system. This review focuses on the emerging role of molecular chaperones in protein quality control, i.e. the mechanism that rids the cell of misfolded or incompletely synthesized polypeptides that otherwise would interfere with normal cellular function.

AAA+ proteins: have engine, will work

The AAA+ (ATPases associated with various cellular activities) family is a large and functionally diverse group of enzymes that are able to induce conformational changes in a wide range of substrate proteins. The family's defining feature is a structurally conserved ATPase domain that assembles into oligomeric rings and undergoes conformational changes during cycles of nucleotide binding and hydrolysis. Here, we review the structural organization of AAA+ proteins, the conformational changes they undergo, the range of different reactions they catalyse, and the diseases associated with their dysfunction.

Asymmetric interactions of ATP with the AAA+ ClpX6 unfoldase: allosteric control of a protein machine

ATP hydrolysis by AAA+ ClpX hexamers powers protein unfolding and translocation during ClpXP degradation. Although ClpX is a homohexamer, positive and negative allosteric interactions partition six potential nucleotide binding sites into three classes with asymmetric properties. Some sites release ATP rapidly, others release ATP slowly, and at least two sites remain nucleotide free. Recognition of the degradation tag of protein substrates requires ATP binding to one set of sites and ATP or ADP binding to a second set of sites, suggesting a mechanism that allows repeated unfolding attempts without substrate release over multiple ATPase cycles. Our results rule out concerted hydrolysis models involving ClpX(6)*ATP(6) or ClpX(6)*ADP(6) and highlight structures of hexameric AAA+ machines with three or four nucleotides as likely functional states. These studies further emphasize commonalities between distant AAA+ family members, including protein and DNA translocases, helicases, motor proteins, clamp loaders, and other ATP-dependent enzymes.

Cytoplasmic degradation of ssrA-tagged proteins

Degradation of ssrA-tagged proteins is a central feature of protein-quality control in all bacteria. In Escherichia coli, the ATP-dependent ClpXP and ClpAP proteases are thought to participate in this process, but their relative contributions to degradation of ssrA-tagged proteins in vivo have been uncertain because two adaptor proteins, ClpS and SspB, can modulate proteolysis of these substrates. Here, intracellular levels of these protease components and adaptors were determined during exponential growth and as cells entered early stationary phase. Levels of ClpA and ClpP increased about threefold during this transition, whereas ClpX, ClpS and SspB levels remained nearly constant. Using GFP-ssrA expressed from the chromosome as a degradation reporter, the effects of altered concentrations of different protease components or adaptor proteins were explored. Both ClpXP and ClpAP degraded GFP-ssrA in the cell, demonstrating that wild-type levels of SspB and ClpS do not inhibit ClpAP completely. Upon entry into stationary phase, increased levels of ClpAP resulted in increased degradation of ssrA-tagged substrates. As measured by maximum turnover rates, ClpXP degradation of GFP-ssrA in vivo was significantly more efficient than in vitro. Surprisingly, ClpX-dependent ClpP-independent degradation of GFP-ssrA was also observed. Thus, unfolding of this substrate by ClpX appears to enhance intracellular degradation by other proteases.

Measuring chromosome dynamics on different time scales using resolvases with varying half-lives

The bacterial chromosome is organized into multiple independent domains, each capable of constraining the plectonemic negative supercoil energy introduced by DNA gyrase. Different experimental approaches have estimated the number of domains to be between 40 and 150. The site-specific resolution systems of closely related transposons Tn3 and gammadelta are valuable tools for measuring supercoil diffusion and analysing bacterial chromosome dynamics in vivo. Once made, the wild-type resolvase persists in cells for time periods greater than the cell doubling time. To examine chromosome dynamics over shorter time frames that are more closely tuned to processes like inducible transcription, we constructed a set of resolvases with cellular half-lives ranging from less than 5 min to 30 min. Analysing chromosomes on different time scales shows domain structure to be dynamic. Rather than the 150 domains detected with the Tn3 resolvase, wild-type cells measured over a 10 min time span have more than 400 domains per genome equivalent, and some gyrase mutants exceed 1000.

The ClpP double ring tetradecameric protease exhibits plastic ring-ring interactions, and the N termini of its subunits form flexible loops that are essential for ClpXP and ClpAP complex formation

ClpP is a conserved serine-protease with two heptameric rings that enclose a large chamber containing the protease active sites. Each ClpP subunit can be divided into a handle region, which mediates ring-ring interactions, and a head domain. ClpP associates with the hexameric ATPases ClpX and ClpA, which can unfold and translocate substrate proteins through the ClpP axial pores into the protease lumen for degradation. We have determined the x-ray structure of Streptococcus pneumoniae ClpP(A153P) at 2.5 A resolution. The structure revealed two novel features of ClpP which are essential for ClpXP and ClpAP functional activities. First, the Ala --> Pro mutation disrupts the handle region, resulting in an altered ring-ring dimerization interface, which, in conjunction with biochemical data, demonstrates the unusual plasticity of this region. Second, the structure shows the existence of a flexible N-terminal loop in each ClpP subunit. The loops line the axial pores in the ClpP tetradecamer and then protrude from the protease apical surface. The sequence of the N-terminal loop is highly conserved in ClpP across all kingdoms of life. These loops are essential determinants for complex formation between ClpP and ClpX/ClpA. Mutation of several amino acid residues in this loop or the truncation of the loop impairs ClpXP and ClpAP complex formation and prevents the coupling between ClpX/ClpA and ClpP activities.

Nucleotide-dependent substrate handoff from the SspB adaptor to the AAA+ ClpXP protease

The SspB adaptor enhances ClpXP degradation by binding the ssrA degradation tag of substrates and the AAA+ ClpX unfoldase. To probe the mechanism of substrate delivery, we engineered a disulfide bond between the ssrA tag and SspB and demonstrated otherwise normal interactions by solving the crystal structure. Although the covalent link prevents adaptor.substrate dissociation, ClpXP degraded GFP-ssrA that was disulfide bonded to the adaptor. Thus, crosslinked substrate must be handed directly from SspB to ClpX. The ssrA tag in the covalent adaptor complex interacted with ClpX.ATPgammaS but not ClpX.ADP, suggesting that handoff occurs in the ATP bound enzyme. By contrast, SspB alone bound ClpX in both nucleotide states. Similar handoff mechanisms will undoubtedly be used by many AAA+ adaptors and enzymes, allowing assembly of delivery complexes in either nucleotide state, engagement of the recognition tag in the ATP state, and application of an unfolding force to the attached protein following hydrolysis.

Sculpting the proteome with AAA(+) proteases and disassembly machines

Machines of protein destruction-including energy-dependent proteases and disassembly chaperones of the AAA(+) ATPase family-function in all kingdoms of life to sculpt the cellular proteome, ensuring that unnecessary and dangerous proteins are eliminated and biological responses to environmental change are rapidly and properly regulated. Exciting progress has been made in understanding how AAA(+) machines recognize specific proteins as targets and then carry out ATP-dependent dismantling of the tertiary and/or quaternary structure of these molecules during the processes of protein degradation and the disassembly of macromolecular complexes.

Thermotolerance Requires Refolding of Aggregated Proteins by Substrate Translocation through the Central Pore of ClpB

Cell survival under severe thermal stress requires the activity of the ClpB (Hsp104) AAA+ chaperone that solubilizes and reactivates aggregated proteins in concert with the DnaK (Hsp70) chaperone system. How protein disaggregation is achieved and whether survival is solely dependent on ClpB-mediated elimination of aggregates or also on reactivation of aggregated proteins has been unclear. We engineered a ClpB variant, BAP, which associates with the ClpP peptidase and thereby is converted into a degrading disaggregase. BAP translocates substrates through its central pore directly into ClpP for degradation. ClpB-dependent translocation is demonstrated to be an integral part of the disaggregation mechanism. Protein disaggregation by the BAP/ClpP complex remains dependent on DnaK, defining a role for DnaK at early stages of the disaggregation reaction. The activity switch of BAP to a degrading disaggregase does not support thermotolerance development, demonstrating that cell survival during severe thermal stress requires reactivation of aggregated proteins.