Properties and structures of the influenza and HIV fusion peptides on lipid membranes: implications for a role in fusion.

The fusion peptides of HIV and influenza virus are crucial for viral entry into a host cell. We report the membrane-perturbing and structural properties of fusion peptides from the HA fusion protein of influenza virus and the gp41 fusion protein of HIV. Our goals were to determine: 1), how fusion peptides alter structure within the bilayers of fusogenic and nonfusogenic lipid vesicles and 2), how fusion peptide structure is related to the ability to promote fusion. Fluorescent probes revealed that neither peptide had a significant effect on bilayer packing at the water-membrane interface, but both increased acyl chain order in both fusogenic and nonfusogenic vesicles. Both also reduced free volume within the bilayer as indicated by partitioning of a lipophilic fluorophore into membranes. These membrane ordering effects were smaller for the gp41 peptide than for the HA peptide at low peptide/lipid ratio, suggesting that the two peptides assume different structures on membranes. The influenza peptide was predominantly helical, and the gp41 peptide was predominantly antiparallel beta-sheet when membrane bound, however, the depths of penetration of Trps of both peptides into neutral membranes were similar and independent of membrane composition. We previously demonstrated: 1), the abilities of both peptides to promote fusion but not initial intermediate formation during PEG-mediated fusion and 2), the ability of hexadecane to compete with this effect of the fusion peptides. Taken together, our current and past results suggest a hypothesis for a common mechanism by which these two viral fusion peptides promote fusion.

Structural analysis of lipoprotein E particles.

Apolipoprotein E (apoE) is a key regulator of cholesterol homeostasis. Human apoE has three common isoforms, each with different risk implications for cardiovascular and neurodegenerative disease. Neither the structure of lipoprotein E particles nor the structural consequences of the isoform differences are known. In this investigation, synthetic lipoprotein particles were prepared by complexing phospholipids with full-length apoE isoforms, or with truncated N-terminal and C-terminal domains of apoE. These particles were examined with calorimetry, electron microscopy, circular dichroism spectroscopy, and internal reflection infrared spectroscopy. Results indicate that particles made with the three full-length apoE isoforms are discoidal in shape, and structurally indistinguishable. Thus, differences in their pathological consequences are not due to gross differences in particle structure. Although apoE is predominantly helical, and the axes of the helices are parallel to the flat surfaces of the particles, the orientational order of lipid acyl chains is low and inconsistent with the belt model of lipoprotein A-I structure. Instead, the data suggest that there are at least two different types of apoE-lipid interactions within lipoprotein E particles. One type occurs between apoE helices and the edge of the lipid bilayer as in the belt model, while a second type involves apoE helices that situate in the plane of the membrane and disturb acyl chain order. These interactions allow LpE particles to form with different protein/lipid ratios, and they account for the structure of LpE particles made with only the truncated domains.

Early synergy between Abeta42 and oxidatively damaged membranes in promoting amyloid fibril formation by Abeta40.

Oxidative lipid membrane damage is known to promote the misfolding of Abeta42 into pathological beta structure. In fully developed senile plaques of Alzheimer's disease, however, it is the shorter and more soluble amyloid beta protein, Abeta40, that predominates. To investigate the role of oxidative membrane damage in the misfolding of Abeta40, we have examined its interaction with supported lipid monolayer membranes using internal reflection infrared spectroscopy. Oxidatively damaged lipids modestly increased Abeta40 accumulation, with adsorption kinetics and a conformation that are distinct from that of Abeta42. In stark contrast, pretreatment of oxidatively damaged monolayer membranes with Abeta42 vigorously promoted Abeta40 accumulation and misfolding. Pretreatment of saturated or undamaged membranes with Abeta42 had no such effect. Parallel studies of lipid bilayer vesicles using a dye binding assay to detect fibril formation and electron microscopy to examine morphology demonstrated that Abeta42 pretreatment of oxidatively damaged membranes promoted the formation of mature Abeta40 amyloid fibrils. We conclude that oxidative membrane damage and Abeta42 act synergistically at an early stage to promote fibril formation by Abeta40. This synergy could be detected within minutes using internal reflection spectroscopy, whereas a dye-binding assay required several days and much higher protein concentrations to demonstrate this synergy.

Role of alpha-synuclein carboxy-terminus on fibril formation in vitro.

Alpha-synuclein (alpha-syn) is the major component of intracellular inclusions in several neurodegenerative diseases, and the conversion of soluble alpha-syn into filamentous aggregates may contribute to disease pathogenesis. Since mechanisms leading to the formation of alpha-syn inclusions are unclear, in vitro models of alpha-syn aggregation may yield insights into this process. To that end, we examined the consequences on the progressive deletion of the carboxy-terminus of alpha-syn in regulating fibril formation, and we show here that carboxy-terminal truncated alpha-syn proteins aggregate faster than the full-length molecule. Protease digestion and immunoelectron microscopy indicate that the alpha-syn amino- and carboxy-termini are more solvent exposed than the central core and that filaments formed from carboxy-terminal truncated alpha-syn are narrower in diameter than the full-length molecule. Moreover, seeding experiments under conditions where full-length alpha-syn did not readily aggregate revealed that carboxy-truncated alpha-syn extending from amino acids 1-102 and 1-110 but not 1-120 were efficient in seeding full-length alpha-syn aggregation over a range of concentrations. Using site-directed mutagenesis, the negatively charged residues 104, 105 and 114, 115 in the carboxy-terminus were implicated in this reduced aggregation and the lack of seeding of full-length alpha-syn fibrillogenesis by 1-120. Our data support the view that the middle region of alpha-syn forms the core of alpha-syn filaments and that negative charges in the carboxy-terminus counteract alpha-syn aggregation. Thus, the carboxy-terminus of alpha-syn may regulate aggregation of full-length alpha-syn and determine the diameter of alpha-syn filaments.

Lipoprotein receptor binding, cellular uptake, and lysosomal delivery of fusions between the receptor-associated protein (RAP) and alpha-L-iduronidase or acid alpha-glucosidase.

Enzyme replacement therapy for lysosomal storage disorders depends on efficient uptake of recombinant enzyme into the tissues of patients. This uptake is mediated by oligosaccharide receptors including the cation-independent mannose 6-phosphate receptor and the mannose receptor. We have sought to exploit alternative receptor systems that are independent of glycosylation but allow for efficient delivery to the lysosome. Fusions of the human lysosomal enzymes alpha-l-iduronidase or acid alpha-glucosidase with the receptor-associated protein were efficiently endocytosed by lysosomal storage disorder patient fibroblasts, rat C6 glioma cells, mouse C2C12 myoblasts, and recombinant Chinese hamster ovary cells expressing individual members of the low-density lipoprotein receptor family. Uptake of the fusions exceeded that of phosphorylated enzyme in all cases, often by an order of magnitude or greater. Uptake was specifically mediated by members of the low-density lipoprotein receptor protein family and was followed by delivery of the fusions to the lysosome. The advantages of the lipoprotein receptor system over oligosaccharide receptor systems include more efficient cellular delivery and the potential for transcytosis of ligands across tight endothelia, including the blood-brain barrier.