Structural and mechanistic characterization of an archaeal-like chaperonin from a thermophilic bacterium.

The chaperonins (CPNs) are megadalton sized hollow complexes with two cavities that open and close to encapsulate non-native proteins. CPNs are assigned to two sequence-related groups that have distinct allosteric mechanisms. In Group I CPNs a detachable co-chaperone, GroES, closes the chambers whereas in Group II a built-in lid closes the chambers. Group I CPNs have a bacterial ancestry, whereas Group II CPNs are archaeal in origin. Here we describe open and closed crystal structures representing a new phylogenetic branch of CPNs. These Group III CPNs are divergent in sequence and structure from extant CPNs, but are closed by a built-in lid like Group II CPNs. A nucleotide-sensing loop, present in both Group I and Group II CPNs, is notably absent. We identified inter-ring pivot joints that articulate during ring closure. These Group III CPNs likely represent a relic from the ancestral CPN that formed distinct bacterial and archaeal branches.Chaperonins (CPNs) are ATP-dependent protein-folding machines. Here the authors present the open and closed crystal structures of a Group III CPN from the thermophilic bacterium Carboxydothermus hydrogenoformans, discuss its mechanism and structurally compare it with Group I and II CPNs.

Quantitative analysis of the impact of a human pathogenic mutation on the CCT5 chaperonin subunit using a proxy archaeal ortholog.

The human chaperonin complex is a ~ 1 MDa nanomachine composed of two octameric rings formed from eight similar but non-identical subunits called CCT. Here, we are elucidating the mechanism of a heritable CCT5 subunit mutation that causes profound neuropathy in humans. In previous work, we introduced an equivalent mutation in an archaeal chaperonin that assembles into two octameric rings like in humans but in which all subunits are identical. We reported that the hexadecamer formed by the mutant subunit is unstable with impaired chaperoning functions. This study quantifies the loss of structural stability in the hexadecamer due to the pathogenic mutation, using differential scanning calorimetry (DSC) and isothermal titration calorimetry (ITC). The disassembly of the wild type complex, which is tightly coupled with subunit denaturation, was decoupled by the mutation without affecting the stability of individual subunits. Our results verify the effectiveness of the homo-hexadecameric archaeal chaperonin as a proxy to assess the impact of subtle defects in heterologous systems with mutations in a single subunit.

Polyanion-tobramycin nanocomplexes into functional microparticles for the treatment of Pseudomonas aeruginosa infections in cystic fibrosis.

AIM: Efficacy of antibiotics in cystic fibrosis (CF) is compromised by the poor penetration through mucus barrier. This work proposes a new 'nano-into-micro' approach, used to obtain a combinatorial effect: achieve a sustained delivery of tobramycin and overcome mucus barrier. METHODS: Mannitol microparticles (MPs) were loaded with a tobramycin polymeric nanocomplex and characterized in presence of CF artificial mucus. RESULTS & DISCUSSION: MPs are able to alter the rheological properties of CF artificial mucus, enhancing drug penetration into it and allowing a prolonged drug release. MPs resulted to be effective in Pseudomonas aeruginosa infections if compared with free tobramycin. CONCLUSION: MPs resulted to be a formulation of higher efficacy, with potential positive implications, as lower required dose, administration frequency, side effects and antibiotic resistance problems.

Hsp60, amateur chaperone in amyloid-beta fibrillogenesis.

BACKGROUND: Molecular chaperones are a very special class of proteins that play essential roles in many cellular processes like folding, targeting and transport of proteins. Moreover, recent evidence indicates that chaperones can act as potentially strong suppressor agents in Alzheimer's disease (AD). Indeed, in vitro experiments demonstrate that several chaperones are able to significantly slow down or suppress aggregation of Aß peptide and in vivo studies reveal that treatment with specific chaperones or their overexpression can ameliorate some distinct pathological signs characterizing AD. METHODS: Here we investigate using a biophysical approach (fluorescence, circular dichroism (CD), transmission electron (TEM) and atomic force (AFM) microscopy, size exclusion chromatography (SEC)) the effect of the human chaperonin Hsp60 on Aß fibrillogenesis. RESULTS: We found that Hsp60 powerfully inhibits Aß amyloid aggregation, by closing molecular pathways leading to peptide fibrillogenesis. CONCLUSIONS: We observe that Hsp60 inhibits Aß aggregation through a more complex mechanism than a simple folding chaperone action. The action is specifically directed toward the early oligomeric species behaving as aggregation seeds for on-pathway amyloid fibrillogenesis. GENERAL SIGNIFICANCE: Understanding the specificity of the molecular interactions of Hsp60 with amyloid Aß peptide allowed us to emphasize the important aspects to be taken into consideration when considering the recent promising therapeutic strategies for neurodegeneration.

Amyloid ß-peptide insertion in liposomes containing GM1-cholesterol domains.

Neuronal membrane damage is related to the early impairments appearing in Alzheimer's disease due to the interaction of the amyloid ß-peptide (Aß) with the phospholipid bilayer. In particular, the ganglioside GM1, present with cholesterol in lipid rafts, seems to be able to initiate Aß aggregation on membrane. We studied the thermodynamic and structural effects of the presence of GM1 on the interaction between Aß and liposomes, a good membrane model system. Isothermal Titration Calorimetry highlighted the importance of the presence of GM1 in recruiting monomeric Aß toward the lipid bilayer. Light and Small Angle X-ray Scattering revealed a different pattern for GM1 containing liposomes, both before and after interaction with Aß. The results suggest that the interaction with GM1 brings to insertion of Aß in the bilayer, producing a structural perturbation down to the internal layers of the liposome, as demonstrated by the obtained electron density profiles.

α-Casein Inhibits Insulin Amyloid Formation by Preventing the Onset of Secondary Nucleation Processes.

α-Casein is known to inhibit the aggregation of several proteins, including the amyloid ß-peptide, by mechanisms that are not yet completely clear. We studied its effects on insulin, a system extensively used to investigate the properties of amyloids, many of which are common to all proteins and peptides. In particular, as for other proteins, insulin aggregation is affected by secondary nucleation pathways. We found that α-casein strongly delays insulin amyloid formation, even at extremely low doses, when the aggregation process is characterized by secondary nucleation. At difference, it has a vanishing inhibitory effect on the initial oligomer formation, which is observed at high concentration and does not involve any secondary nucleation pathway. These results indicate that an efficient inhibition of amyloid formation can be achieved by chaperone-like systems, by sequestering the early aggregates, before they can trigger the exponential proliferation brought about by secondary nucleation mechanisms.