Common motifs in protein self-assembly

Abstract

The importance of misfolding proteins forming amyloid fibrils for the aetiology of many diseases, particularly those of old age, is well recognized. This phenomenon is now thought to be a universal property of proteins, as long as appropriate conditions for loosening the native folded structure can be found, which may be outside those of normal physiology. However, the beta-sheet-rich structure of the amyloid fibril does not need to exist in isolation. Recent work has shown that higher order assemblies of the fibrils occur into structures resembling spherulites found in common synthetic semi-crystalline polymers. In these, the fibrils grow outwards from an inner core, thought to be amorphous. Data will be presented on the kinetics of growth of these fibrils for different proteins, so that similarities and differences can be revealed, and related to subtle differences in appearance under the microscope. The in vitro assembly of amyloid fibrils is usually thought to occur well away from the isoelectric point of the protein, and these are the conditions under which they have most been studied. Around the isoelectric point, particulate self-assembly is known to occur for beta-lactoglobulin, and we can now show this is also a generic form of protein self-assembly once the net charge on the protein is close to zero. Nevertheless, the charge is not actually zero, and salt in the solution is found to have a significant effect on the growth of the particles. The use of SAXS, thioflavin T staining and FTIR also shows that within the particles there is also clear evidence for amyloid-like beta-sheet structure, particularly in the case when salt is absent, demonstrating that this particular motif underlies this very different form of protein self-assembly.

Nonnative Aggregation of an IgG1 Antibody in Acidic Conditions: Part 1. Unfolding, Colloidal Interactions, and Formation of High-Molecular-Weight Aggregates

“…15 For small proteins and peptides, aggregates are often categorized as either amyloid or nonamyloid; the latter are also often described as "amorphous" to indicate that they lack the filamentous morphology and specific beta-sheet and dye-binding motifs that are characteristic of amyloid aggregates. 4,24,25 Recent results for non-MAb proteins and polypeptides illustrated that different aggregate states can be formed by the same protein, depending on the solution conditions of aggregate formation. [24][25][26][27][28] Conditions that provided weakly attractive colloidal interactions correlated well with amyloid 24 and/or insoluble aggregate formation; 24,27 conversely, conditions with strongly repulsive electrostatic interactions led to nonamyloid aggregates in some cases.…”

“…4,24,25 Recent results for non-MAb proteins and polypeptides illustrated that different aggregate states can be formed by the same protein, depending on the solution conditions of aggregate formation. [24][25][26][27][28] Conditions that provided weakly attractive colloidal interactions correlated well with amyloid 24 and/or insoluble aggregate formation; 24,27 conversely, conditions with strongly repulsive electrostatic interactions led to nonamyloid aggregates in some cases. 24 In this context, colloidal interactions denote screened electrostatic attractions and repulsions, van der Waals and hydrophobic attractions, and steric repulsions; the combination of forces may be net attractive or repulsive, but typically is not strong enough to create highly specific "binding" sites between two proteins such as might be found for a natively dimeric or multimeric protein.…”

“…[24][25][26][27][28] Conditions that provided weakly attractive colloidal interactions correlated well with amyloid 24 and/or insoluble aggregate formation; 24,27 conversely, conditions with strongly repulsive electrostatic interactions led to nonamyloid aggregates in some cases. 24 In this context, colloidal interactions denote screened electrostatic attractions and repulsions, van der Waals and hydrophobic attractions, and steric repulsions; the combination of forces may be net attractive or repulsive, but typically is not strong enough to create highly specific "binding" sites between two proteins such as might be found for a natively dimeric or multimeric protein. 29 Additionally, both theoretical analysis 15 and experimental studies 30 suggest that MAbs may be capable of forming amyloid-like aggregates.…”

Small, Stable, and Monodispersed Bubbles Encapsulated with Biopolymers

“…This pattern was earlier reported for the spherical protein aggregates formed at low pH and elevated temperature, which featured the radial arrangement of amyloid fibrils with cross b-sheet internal structure. [35,36] However, the dimensions of the spherical aggregates on the surface of the bubble (as in Figure 4D) were only 500-700 nm, corresponding to short fragments of amyloid fibrils. [37] Furthermore, to verify the existence of fragments of amyloid fibrils in the biopolymer shell of the bubbles stored for 28 d, we performed a Thioflavin-T (ThT) binding test.…”

“…In comparison with a freshly prepared lysozyme-ThT solution, alginate-ThT solution and a stock ThT solution, we observed the enhancement of fluorescence intensity ( Figure 5B), which confirmed the existence of fragmented amyloid fibrils on the bubble surface. [35,38] …”