Macromolecular Crowding Accelerates Amyloid Formation by Human Apolipoprotein C-II

Abstract

The specific self-association of proteins to form amyloid fibrils is a characteristic of a number of pathologies, including Alzheimer, Parkinson, and Creutzfeldt-Jakob diseases (1). This process involves slow nucleation coupled to self-association steps, which constitute an alternative folding pathway to those leading to the native state (2, 3). Amyloid formation is promoted by destabilization of the native state through events such as mutation or truncation. For example, certain mutants of lysozyme that exhibit molten globule characteristics also form amyloid (4) whereas apomyoglobin forms amyloid under partially denaturing conditions (5).Human apolipoprotein C-II (apoC-II) 1 (M r ϭ 8915, 79 residues) is normally a component of very low density lipoprotein, where it plays an important physiological role as an activator of lipoprotein lipase (6, 7). When associated with polar lipids (e.g. phospholipids or SDS) apoC-II adopts a primarily ␣-helical conformation (8 -10). Our previous work (11) demonstrates that, in the absence of lipid, human apoC-II self-associates to form twisted ribbon-like fibrils with all of the hallmarks of amyloid, including binding to Congo Red with red-green birefringence under cross-polarized light, binding to thioflavin T, and increased ␤-structure. In addition, x-ray diffraction patterns of aligned apoC-II fibrils indicate a cross-␤-sheet structure.2 In vitro amyloid formation by apoC-II can be compared with the in vivo deposition of amyloid involving other apolipoproteins such as apoA-I (12, 13), apoA-II (14, 15), apoA-IV (16), apoE (17), and apolipoprotein-like proteins, ␣-synuclein (18) and serum amyloid A (19). A significant clue to the prevalence of apolipoproteins in amyloid formation is provided by the observation that many apolipoproteins have limited conformational stability or secondary structure in the absence of lipid (20). Destabilized conformations in many proteins promote amyloid formation (3-5, 21). Apolipoprotein derivatives that form amyloid are frequently mutant isoforms or truncated products; for example, amyloid deposition involving apoA-I (12), apoA-II (14), and the C-terminal domain of apoE (17). We propose that these modifications destabilize lipid binding leading to amyloid formation in vivo (20). The well-characterized ability of apoC-II to form amyloid fibrils provides a convenient model to examine in vivo parameters (9, 22) that could control the growth of amyloid fibrils in vivo.Most biological fluids contain a high total concentration of macromolecules, including proteins, nucleic acids, and carbohydrates, that collectively occupy a high fraction of the fluid volume (23). Volume exclusion or "macromolecular crowding" in such fluids may result in significant alterations of the rates and equilibria of macromolecular associations (23-26). It has been suggested on conceptual grounds that volume exclusion in physiological media could modulate the rate and extent of amyloid formation in vivo (27,28). In the present study we investigate the potential effects of ma...

How can biochemical reactions within cells differ from those in test tubes?

“…(Drenckhahn and Pollard, 1986), tubulin (Herzog and Weber, 1978) and fibrin (Wilf et al, 1985). Large increases in the rate of amyloid formation by apoCII (Hatters et al, 2002) and ␣-synuclein (Shtilerman et al, 2002;Uversky et al, 2002). Large increases in the rate of self-assembly of HIV capsid protein (del Alamo et al, 2005).…”

Protein–protein interactions in a crowded environment

“…Previous successes in this direction indicate that such an approximation is acceptable in several cases (Minton 1983;Ellis 2001;Hatters et al 2002;Ellis and Minton 2006;Stagg et al 2007), and in fact one can even come up with rational ways to define effective hard-sphere diameter for proteins (Kim et al 2010;Mittal and Best 2010). On the other hand, the failure of the SPT to explain the crowding data does not necessarily invalidate the SPT or the underlying spherical approximation.…”

New Class of Inhibitors of Amyloid-β Fibril Formation

“…The S values represent the predominant populations of oligomeric A␤1-40 species as determined by fitting the experimental data (the sedimentation coefficient distribution plots g(s*)) to a one-, two-, or three-species model using a Gaussian function. observation and characterization of soluble HMW oligomeric intermediates formed during fibrillogenesis for several amyloidogenic proteins including A␤ (34,(43)(44)(45)(46)(47). In this paper, analytical ultracentrifugation and EM were employed to probe the mechanism by which catecholamines such as apomorphine and its derivatives interact with A␤ and inhibit fibril formation in vitro.…”