From hexamer to amyloid: marginal stability of apolipoprotein SAA2.2 leads to in vitro fibril formation at physiological temperature

Nature of recombinant human serum amyloid A1 in Escherichia coli and its preferable approach for purification

Serum amyloid A1 (SAA1) is an apolipoprotein which is involved in amyloid A amyloidosis (AA) by forming fibrils. The process of fibrillation is still being explored and holds challenges in recombinant expression and purification of SAA1. This study deals with the preferable approach for the expression and purification of SAA1 which is normally toxic and unstable to express without using any fusion-tag. Complete soluble expression of SAA1 was obtained without the use of additional tag, in terrific broth, supplemented with 3 % ethanol at 30 °C. Soluble fraction of SAA1 was initially treated with salting-out using ammonium sulphate giving 1.5 M salt concentration to avoid SAA1 protein precipitation along with unwanted proteins. The soluble fraction of SAA1 after salting-out was purified by two individual chromatographic approaches: One anion exchange and second reverse phase chromatography. The yield of purified SAA1 was 3 times greater by anion exchange than reverse phase chromatography. MALDI-TOF analysis of purified SAA1 showed 11813 Da for intact protein and proteome analysis revealed greater than 90 % sequence coverage by MASCOT. The subunit interaction showed hexamer form at basic pH which was analyzed by size exclusion chromatography. The fibrillation activity of SAA1 was found to be 10-15 times higher in basic media at 43 °C than 37 °C. Our research demonstrates successful expression and purification of wild-type human recombinant SAA1. The cost-effective radical approach employed for purification of SAA1 is crucial for thorough protein characterization particularly, mechanisms of protein aggregation involved in amyloidosis.

Authentic hSAA related with AA amyloidosis: New method of purification, folding and amyloid polymorphism

The secondary amyloidosis of humans is caused by the formation of hSAA fibrils in different organs and tissues. Until now hSAA was thought to have low amyloidogenicity in vitro and the majority of SAA aggregation experiments were done using murine protein or hSAA non-pathogenic isoforms. In this work a novel purification method for recombinant hSAA was introduced, enabling to obtain monomeric protein capable of amyloid aggregation under physiological conditions. The stability and amyloid aggregation of hSAA have been examined using a wide range of biophysical methods. It was shown that the unfolding of monomeric protein occurs through the formation of molten globule-like intermediate state. Polymorphism of hSAA amyloids was discovered to depend on the solution pH. At pH 8.5, rapid protein aggregation occurs, which leads to the formation of twisted short fibrils. Even a slight decrease of the pH to 7.8 results in delayed aggregation with the formation of long straight amyloids composed of laterally associated protofilaments. Limited proteolysis experiments have shown that full-length hSAA is involved in the formation of intermolecular interactions in both amyloid polymorphs. The results obtained, and the experimental approach used in this study can serve as a basis for further research on the mechanism of authentic hSAA amyloid formation.

Acute-serum amyloid A and A-SAA-derived peptides as formyl peptide receptor (FPR) 2 ligands

Originally, it was thought that a single serum amyloid A (SAA) protein was involved in amyloid A amyloidosis, but in fact, SAA represents a four-membered family wherein SAA1 and SAA2 are acute phase proteins (A-SAA). SAA is highly conserved throughout evolution within a wide range of animal species suggestive of an important biological function. In fact, A-SAA has been linked to a number of divergent biological activities wherein a number of these functions are mediated via the G protein-coupled receptor (GPCR), formyl peptide receptor (FPR) 2. For instance, through the activation of FPR2, A-SAA has been described to regulate leukocyte activation, atherosclerosis, pathogen recognition, bone formation and cell survival. Moreover, A-SAA is subject to post-translational modification, primarily through proteolytic processing, generating a range of A-SAA-derived peptides. Although very little is known regarding the biological effect of A-SAA-derived peptides, they have been shown to promote neutrophil and monocyte migration through FPR2 activation via synergy with other GPCR ligands namely, the chemokines CXCL8 and CCL3, respectively. Within this review, we provide a detailed analysis of the FPR2-mediated functions of A-SAA. Moreover, we discuss the potential role of A-SAA-derived peptides as allosteric modulators of FPR2.

Uncovering the universality of self-replication in protein aggregation and its link to disease

Fibrillar protein aggregates are a hallmark of a range of human disorders, from prion diseases to dementias, but are also encountered in several functional contexts. Yet, the fundamental links between protein assembly mechanisms and their functional or pathological roles have remained elusive. Here, we analyze the aggregation kinetics of a large set of proteins that self-assemble by a nucleated-growth mechanism, from those associated with disease, over those whose aggregates fulfill functional roles in biology, to those that aggregate only under artificial conditions. We find that, essentially, all such systems, regardless of their biological role, are capable of self-replication. However, for aggregates that have evolved to fulfill a structural role, the rate of self-replication is too low to be significant on the biologically relevant time scale. By contrast, all disease-related proteins are able to self-replicate quickly compared to the time scale of the associated disease. Our findings establish the ubiquity of self-replication and point to its potential importance across aggregation-related disorders.

Understanding the role of hydrophobic patches in protein disaggregation

Protein folding is a very complex process and, so far, the mechanism of folding still intrigues the research community. Despite a large conformational space available (O(10⁴⁷) for a 100 amino acid residue), most proteins fold into their native state within a very short time. While small proteins fold relatively fast (a few microseconds) large globular proteins may take as long as several milliseconds to fold. During the folding process, the protein synthesized in the ribosome is exposed to the crowded environment of the cell and is easily prone to misfolding and aggregation due to interactions with other proteins or biomacromolecules present within the cell. These large proteins, therefore, rely on chaperones for their folding and repair. Chaperones are known to have hydrophobic patchy domains that play a crucial role in shielding the protein against misfolding and disaggregation of aggregated proteins. In the current article, Monte Carlo simulations carried out in the framework of the hydrophobic-polar (H-P) lattice model indicate that hydrophobic patchy domains drastically reduce the inter-protein interactions and are efficient in disaggregating proteins. The effectiveness of the disaggregation depends on the size and distribution of these patches on the surface and also on the strength of the interaction between the protein and the surface. Further, our results indicate that when the patch is complementary to the exposed hydrophobic patch of the protein, protein disaggregation is accompanied by stabilization of the protein even relative to its bulk behavior due to favorable protein-surface interactions. We believe that these findings shed light on the role of the class of chaperones known as heat shock proteins (Hsps) on protein disaggregation and refolding.

The turning away of serum amyloid A biological activities and receptor usage

Serum amyloid A (SAA) is an acute-phase protein (APP) to which multiple immunological functions have been attributed. Regardless, the true biological role of SAA remains poorly understood. SAA is remarkably conserved in mammalian evolution, thereby suggesting an important biological function. Since its discovery in the 1970s, the majority of researchers have investigated SAA using recombinant forms made available through bacterial expression. Nevertheless, recent studies indicate that these recombinant forms of SAA are unreliable. Indeed, commercial SAA variants have been shown to be contaminated with bacterial products including lipopolysaccharides and lipoproteins. As such, biological activities and receptor usage (TLR2, TLR4) revealed through the use of commercial SAA variants may not reflect the inherent nature of this APP. Within this review, we discuss the biological effects of SAA that have been demonstrated through more solid experimental approaches. SAA takes part in the innate immune response via the recruitment of leucocytes and executes, through pathogen recognition, antimicrobial activity. Knockout animal models implicate SAA in a range of functions, such as regulation of T-cell-mediated responses and monopoiesis. Moreover, through its structural motifs, not only does SAA function as an extracellular matrix protein, but it also binds extracellular matrix proteins. Finally, we here also provide an overview of definite SAA receptor-mediated functions and highlight those that are yet to be validated. The role of FPR2 in SAA-mediated leucocyte recruitment has been confirmed; nevertheless, SAA has been linked to a range of other receptors including CD36, SR-BI/II, RAGE and P2RX7.

Influence of short peptides with aromatic amino acid residues on aggregation properties of serum amyloid A and its fragments

Serum amyloid A variant 1.1 (SAA1.1) is an acute phase protein. In response to injury, inflammation or infection its production increases highly, which may lead to aggregation of the protein and accumulation of its deposits in various organs. Due to the cellular toxicity of the aggregates, as well as the fact that accumulated deposits are a burden that obstructs proper functioning of the affected tissues, it is vital to find a way to suppress the process of pathological aggregates formation. To make this possible, it is necessary to investigate thoroughly the oligomerization process and recognize factors that may influence its course. Some previous studies showed that aromatic interactions are important to the potential of an inhibitor to suppress the aggregation process. In our research we had proved that a five-residue peptide RSFFS (saa1-5) is an efficient inhibitor of aggregation of the most amyloidogenic fragment of SAA1.1, SAA1-12. In the present work the oligomerization and aggregation propensity of SAA1-12 was compared to that of SAA1-27, in order to determine the contribution of the sequence which extends beyond the most amyloidogenic region but encompasses residues reportedly involved in the stabilization of the SAA native conformation. Thioflavin T fluorescence assay, quantitative chromatographic analysis of the insoluble fraction and transmission electron microscopy allowed for a deeper insight into the SAA aggregation process and the morphology of aggregates. Substitutions of Phe3 and/or Phe4 residues in saa1-5 sequence with tryptophan, tyrosine, homophenylalanine, naphthylalanine and β,β-diphenylalanine allowed to study the influence of different aromatic systems on the aggregation of SAA1-12 and SAA1-27, and evaluate these results in relation to hSAA1.1 protein. Our results indicate that compounds with aromatic moieties can affect the course of the aggregation process and change the ratio between the soluble and insoluble aggregates.

Serum amyloid A - a review

Serum amyloid A (SAA) proteins were isolated and named over 50 years ago. They are small (104 amino acids) and have a striking relationship to the acute phase response with serum levels rising as much as 1000-fold in 24 hours. SAA proteins are encoded in a family of closely-related genes and have been remarkably conserved throughout vertebrate evolution. Amino-terminal fragments of SAA can form highly organized, insoluble fibrils that accumulate in “secondary” amyloid disease. Despite their evolutionary preservation and dynamic synthesis pattern SAA proteins have lacked well-defined physiologic roles. However, considering an array of many, often unrelated, reports now permits a more coordinated perspective. Protein studies have elucidated basic SAA structure and fibril formation. Appreciating SAA’s lipophilicity helps relate it to lipid transport and metabolism as well as atherosclerosis. SAA’s function as a cytokine-like protein has become recognized in cell-cell communication as well as feedback in inflammatory, immunologic, neoplastic and protective pathways. SAA likely has a critical role in control and possibly propagation of the primordial acute phase response. Appreciating the many cellular and molecular interactions for SAA suggests possibilities for improved understanding of pathophysiology as well as treatment and disease prevention.

Amyloid-Forming Properties of Human Apolipoproteins: Sequence Analyses and Structural Insights

Apolipoproteins are protein constituents of lipoproteins that transport cholesterol and fat in circulation and are central to cardiovascular health and disease. Soluble apolipoproteins can transiently dissociate from the lipoprotein surface in a labile free form that can misfold, potentially leading to amyloid disease. Misfolding of apoA-I, apoA-II, and serum amyloid A (SAA) causes systemic amyloidoses, apoE4 is a critical risk factor in Alzheimer's disease, and apolipoprotein misfolding is also implicated in cardiovascular disease. To explain why apolipoproteins are over-represented in amyloidoses, it was proposed that the amphipathic α-helices, which form the lipid surface-binding motif in this protein family, have high amyloid-forming propensity. Here, we use 12 sequence-based bioinformatics approaches to assess amyloid-forming potential of human apolipoproteins and to identify segments that are likely to initiate β-aggregation. Mapping such segments on the available atomic structures of apolipoproteins helps explain why some of them readily form amyloid while others do not. Our analysis shows that nearly all amyloidogenic segments: (i) are largely hydrophobic, (ii) are located in the lipid-binding amphipathic α-helices in the native structures of soluble apolipoproteins, (iii) are predicted in both native α-helices and β-sheets in the insoluble apoB, and (iv) are predicted to form parallel in-register β-sheet in amyloid. Most of these predictions have been verified experimentally for apoC-II, apoA-I, apoA-II and SAA. Surprisingly, the rank order of the amino acid sequence propensity to form amyloid (apoB>apoA-II>apoC-II≥apoA-I, apoC-III, SAA, apoC-I>apoA-IV, apoA-V, apoE) does not correlate with the proteins' involvement in amyloidosis. Rather, it correlates directly with the strength of the protein-lipid association, which increases with increasing protein hydrophobicity. Therefore, the lipid surface-binding function and the amyloid-forming propensity are both rooted in apolipoproteins' hydrophobicity, suggesting that functional constraints make it difficult to completely eliminate pathogenic apolipoprotein misfolding. We propose that apolipoproteins have evolved protective mechanisms against misfolding, such as the sequestration of the amyloidogenic segments via the native protein-lipid and protein-protein interactions involving amphipathic α-helices and, in case of apoB, β-sheets.

Intrinsic Stability, Oligomerization, and Amyloidogenicity of HDL-Free Serum Amyloid A

Serum amyloid A (SAA) is an acute-phase reactant protein predominantly bound to high-density lipoprotein in serum and presumed to play various biological and pathological roles. Upon tissue trauma or infection, hepatic expression of SAA increases up to 1,000 times the basal levels. Prolonged increased levels of SAA may lead to amyloid A (AA) amyloidosis, a usually fatal systemic disease in which the amyloid deposits are mostly comprised of the N-terminal 1-76 fragment of SAA. SAA isoforms may differ across species in their ability to cause AA amyloidosis, and the mechanism of pathogenicity remains poorly understood. In vitro studies have shown that SAA is a marginally stable protein that folds into various oligomeric species at 4 °C. However, SAA is largely disordered at 37 °C, reminiscent of intrinsically disordered proteins. Non-pathogenic murine (m)SAA2.2 spontaneously forms amyloid fibrils in vitro at 37 °C whereas pathogenic mSAA1.1 has a long lag (nucleation) phase, and eventually forms fibrils of different morphology than mSAA2.2. Remarkably, human SAA1.1 does not form mature fibrils in vitro. Thus, it appears that the intrinsic amyloidogenicity of SAA is not a key determinant of pathogenicity, and that other factors, including fibrillation kinetics, ligand binding effects, fibril stability, nucleation efficiency, and SAA degradation may play key roles. This chapter will focus on the known structural and biophysical properties of SAA and discuss how these properties may help better understand the molecular mechanism of AA amyloidosis.

Divergent effect of glycosaminoglycans on the in vitro aggregation of serum amyloid A

Serum amyloid A (SAA) is an apolipoprotein involved in poorly understood roles in inflammation. Upon trauma, hepatic expression of SAA rises 1000 times the basal levels. In the case of inflammatory diseases like rheumatoid arthritis, there is a risk for deposition of SAA fibrils in various organs leading to Amyloid A (AA) amyloidosis. Although the amyloid deposits in AA amyloidosis accumulate with the glycosaminoglycan (GAG) heparan sulfate, the role GAGs play in the function and pathology of SAA is an enigma. It has been shown that GAG sulfation is a contributing factor in protein fibrillation and for co-aggregating with a plethora of amyloidogenic proteins. Herein, the effects of heparin, heparan sulfate, hyaluronic acid, chondroitin sulfate A, and heparosan on the oligomerization and aggregation properties of pathogenic mouse SAA1.1 were investigated. Delipidated SAA was used to better understand the interactions between SAA and GAGs without the complicating involvement of lipids. The results revealed-to varying degrees-that all GAGs accelerated SAA1.1 aggregation, but had variable effects on its fibrillation. Heparan sulfate, hyaluronic acid, and heparosan did not affect much the fibrillation of SAA1.1. In contrast, chondroitin sulfate A blocked SAA fibril formation and facilitated the formation of spherical aggregates of various sizes. Interestingly, heparin caused formation of spherical SAA1.1 aggregates of various sizes, vast amounts of thin protofibrils, and few long fibrils of various heights. These results suggest that GAGs may have an intrinsic and divergent influence on the aggregation and fibrillation of HDL-free SAA1.1 in vivo, with functional and pathological implications.

Structural mechanism of serum amyloid A-mediated inflammatory amyloidosis

Serum amyloid A (SAA) represents an evolutionarily conserved family of inflammatory acute-phase proteins. It is also a major constituent of secondary amyloidosis. To understand its function and structural transition to amyloid, we determined a structure of human SAA1.1 in two crystal forms, representing a prototypic member of the family. Native SAA1.1 exists as a hexamer, with subunits displaying a unique four-helix bundle fold stabilized by its long C-terminal tail. Structure-based mutational studies revealed two positive-charge clusters, near the center and apex of the hexamer, that are involved in SAA association with heparin. The binding of high-density lipoprotein involves only the apex region of SAA and can be inhibited by heparin. Peptide amyloid formation assays identified the N-terminal helices 1 and 3 as amyloidogenic peptides of SAA1.1. Both peptides are secluded in the hexameric structure of SAA1.1, suggesting that the native SAA is nonpathogenic. Furthermore, dissociation of the SAA hexamer appears insufficient to initiate amyloidogenic transition, and proteolytic cleavage or removal of the C-terminal tail of SAA resulted in formation of various-sized structural aggregates containing ∼5-nm regular repeating protofibril-like units. The combined structural and functional studies provide mechanistic insights into the pathogenic contribution of glycosaminoglycan in SAA1.1-mediated AA amyloid formation.

Characterization of the oligomerization and aggregation of human Serum Amyloid A

The fibrillation of Serum Amyloid A (SAA) – a major acute phase protein – is believed to play a role in the disease Amyloid A (AA) Amyloidosis. To better understand the amyloid formation pathway of SAA, we characterized the oligomerization, misfolding, and aggregation of a disease-associated isoform of human SAA – human SAA1.1 (hSAA1.1) – using techniques ranging from circular dichroism spectroscopy to atomic force microscopy, fluorescence spectroscopy, immunoblot studies, solubility measurements, and seeding experiments. We found that hSAA1.1 formed alpha helix-rich, marginally stable oligomers in vitro on refolding and cross-beta-rich aggregates following incubation at 37°C. Strikingly, while hSAA1.1 was not highly amyloidogenic in vitro, the addition of a single N-terminal methionine residue significantly enhanced the fibrillation propensity of hSAA1.1 and modulated its fibrillation pathway. A deeper understanding of the oligomerization and fibrillation pathway of hSAA1.1 may help elucidate its pathological role.

Pathogenic serum amyloid A 1.1 shows a long oligomer-rich fibrillation lag phase contrary to the highly amyloidogenic non-pathogenic SAA2.2

Serum amyloid A (SAA) is best known for being the main component of amyloid in the inflammation-related disease amyloid A (AA) amyloidosis. Despite the high sequence identity among different SAA isoforms, not all SAA proteins are pathogenic. In most mouse strains, the AA deposits mostly consist of SAA1.1. Conversely, the CE/J type mouse expresses a single non-pathogenic SAA2.2 protein that is 94% identical to SAA1.1. Here we show that SAA1.1 and SAA2.2 differ in their quaternary structure, fibrillation kinetics, prefibrillar oligomers, and fibril morphology. At 37 °C and inflammation-related SAA concentrations, SAA1.1 exhibits an oligomer-rich fibrillation lag phase of a few days, whereas SAA2.2 shows virtually no lag phase and forms small fibrils within a few hours. Deep UV resonance Raman, far UV-circular dichroism, atomic force microscopy, and fibrillation cross-seeding experiments suggest that SAA1.1 and SAA2.2 fibrils possess different morphology. Both the long-lived oligomers of pathogenic SAA1.1 and the fleeting prefibrillar oligomers of non-pathogenic SAA2.2, but not their respective amyloid fibrils, permeabilized synthetic bilayer membranes in vitro. This study represents the first comprehensive comparison between the biophysical properties of SAA isoforms with distinct pathogenicities, and the results suggest that structural and kinetic differences in the oligomerization-fibrillation of SAA1.1 and SAA2.2, more than their intrinsic amyloidogenicity, may contribute to their diverse pathogenicity.

Influence of the carboxy terminus of serum amyloid A on protein oligomerization, misfolding, and fibril formation

The fibrillar deposition of serum amyloid A (SAA) has been linked to the disease amyloid A (AA) amyloidosis. We have used the SAA isoform, SAA2.2, from the CE/J mouse strain, as a model system to explore the inherent structural and biophysical properties of SAA. Despite its nonpathogenic nature in vivo, SAA2.2 spontaneously forms fibrils in vitro, suggesting that SAA proteins are inherently amyloidogenic. However, whereas the importance of the amino terminus of SAA for fibril formation has been well documented, the influence of the proline-rich and presumably disordered carboxy terminus remains poorly understood. To clarify the inherent role of the carboxy terminus in the oligomerization and fibrillation of SAA, we truncated the proline-rich final 13 residues of SAA2.2. We found that unlike full-length SAA2.2, the carboxy-terminal truncated SAA2.2 (SAA2.2ΔC) did not oligomerize to a hexamer or octamer, but formed a high molecular weight soluble aggregate. Moreover, SAA2.2ΔC also exhibited a pronounced decrease in the rate of fibril formation. Intriguingly, when equimolar amounts of denatured SAA2.2 and SAA2.2ΔC were mixed and allowed to refold together, the mixture formed an octamer and exhibited rapid fibrillation kinetics, similar to those for full-length SAA2.2. These results suggest that the carboxy terminus of SAA, which is highly conserved among SAA sequences in all vertebrates, might play important structural roles, including modulating the folding, oligomerization, misfolding, and fibrillation of SAA.

Inflammation protein SAA2.2 spontaneously forms marginally stable amyloid fibrils at physiological temperature

For nearly four decades, the formation of amyloid fibrils by the inflammation-related protein serum amyloid A (SAA) has been pathologically linked to the disease amyloid A (AA) amyloidosis. However, here we show that the nonpathogenic murine SAA2.2 spontaneously forms marginally stable amyloid fibrils at 37 °C that exhibit cross-beta structure, binding to thioflavin T, and fibrillation by a nucleation-dependent seeding mechanism. In contrast to the high stability of most known amyloid fibrils to thermal and chemical denaturation, experiments monitored by glutaraldehyde cross-linking/SDS-PAGE, thioflavin T fluorescence, and light scattering (OD(600)) showed that the mature amyloid fibrils of SAA2.2 dissociate upon incubation in >1.0 M urea or >45 °C. When considering the nonpathogenic nature of SAA2.2 and its ~1000-fold increased concentration in plasma during an inflammatory response, its extreme in vitro amyloidogenicity under physiological-like conditions suggest that SAA amyloid might play a functional role during inflammation. Of general significance, the combination of methods used here is convenient for exploring the stability of amyloid fibrils that are sensitive to urea and temperature. Furthermore, our studies imply that analogous to globular proteins, which can possess structures ranging from intrinsically disordered to extremely stable, amyloid fibrils formed in vivo might have a broader range of stabilities than previously appreciated with profound functional and pathological implications.

Serum amyloid A 2.2 refolds into a octameric oligomer that slowly converts to a more stable hexamer

Serum amyloid A (SAA) is an inflammatory protein predominantly bound to high-density lipoprotein in plasma and presumed to play various biological and pathological roles. We previously found that the murine isoform SAA2.2 exists in aqueous solution as a marginally stable hexamer at 4-20°C, but becomes an intrinsically disordered protein at 37°C. Here we show that when urea-denatured SAA2.2 is dialyzed into buffer (pH 8.0, 4°C), it refolds mostly into an octameric species. The octamer transitions to the hexameric structure upon incubation from days to weeks at 4°C, depending on the SAA2.2 concentration. Thermal denaturation of the octamer and hexamer monitored by circular dichroism showed that the octamer is ∼10°C less stable, with a denaturation mid point of ∼22°C. Thus, SAA2.2 becomes kinetically trapped by refolding into a less stable, but more kinetically accessible octameric species. The ability of SAA2.2 to form different oligomeric species in vitro along with its marginal stability, suggest that the structure of SAA might be modulated in vivo to form different biologically relevant species.

The acute-phase protein serum amyloid A3 is expressed in the bovine mammary gland and plays a role in host defence

The serum amyloid A protein is one of the major reactants in the acute-phase response. Using representational difference analysis comparing RNA from normal and involuting quarters of a dairy cow mammary gland, we found an mRNA encoding the SAA3 protein (M-SAA3). The M-SAA3 mRNA was localized to restricted populations of bovine mammary epithelial cells (MECs). It was expressed at a moderate level in late pregnancy, at a low level through lactation, was induced early in milk stasis, and expressed at high levels in most MECs during mid to late involution and inflammation/mastitis. The mature M-SAA3 peptide was expressed in Escherichia coli, antibodies made, and shown to have antibacterial activity against E. coli, Streptococcus uberis and Pseudomonas aeruginosa. These results suggest that the mammary SAA3 may have a role in protection of the mammary gland during remodelling and infection and possibly in the neonate gastrointestinal tract.

Urea-induced denaturation of apolipoprotein serum amyloid A reveals marginal stability of hexamer

Serum Amyloid A (SAA) is an acute phase reactant protein that is predominantly found bound to high-density lipoprotein in plasma. Upon inflammation, the plasma concentration of SAA can increase dramatically, occasionally leading to the development of amyloid A (AA) amyloidosis, which involves the deposition of SAA amyloid fibrils in major organs. We previously found that the murine isoform SAA2.2 exists in aqueous solution as a hexamer containing a central channel. Here we show using various biophysical and biochemical techniques that the SAA2.2 hexamer can be totally dissociated into monomer by approximately 2 M urea, with the concerted loss of its alpha-helical structure. However, limited trypsin proteolysis experiments in urea showed a conserved digestion profile, suggesting the preservation of major backbone topological features in the urea-denatured state of SAA2.2. The marginal stability of hexameric SAA2.2 and the presence of residual structure in the denatured monomeric protein suggest that both forms may interconvert in vivo to exert different functions to meet the various needs during normal physiological conditions and in response to inflammatory stimuli.