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Despite the rapid progress of the AIDS pandemic, the HIV virus is a surprisingly weak pathogen in vitro. The large difference between in vitro and in vivo infection rates suggests that cofactors absent in vitro but essential for the natural transmission of the virus may be responsible for this discrepancy. A recently identified peptide in human semen, PAP248-286, has emerged as a clear candidate for the missing cofactor as it dramatically enhances the infectivity of HIV by up to five orders of magnitude. PAP248-286 appears to enhance HIV infection by forming amyloid fibers known as SEVI, which are believed to enhance the attachment of the virus by bridging interactions between virion and host-cell membranes. To understand the unique ability of SEVI to enhance HIV infection, we have solved the atomic-level resolution structure of the SEVI precursor PAP248-286 using NMR spectroscopy in SDS micelles. In contrast to other toxic amyloid peptides that generally penetrate into the core of the membrane, non-toxic PAP248-286 binds superficially to the surface of the micelle. Unlike most amyloid peptides that bind to the membrane in an α-helical state, PAP248-286 is mostly disordered when bound to the surface of the micelle. The highly disordered nature of the SEVI peptide may explain the high ability of SEVI to enhance HIV infection, as partially disordered amyloid fibers will have a greater capture radius for the virus than more compact amyloid fibers. Two regions of nascent structure match the prediction of highly amyloidogenic sequences and may serve as nuclei for aggregation and amyloid fibril formation. NMR studies of the binding of PAP248-286 to the anti-amyloid agent ECGC will also be presented.

Despite the rapid progress of the AIDS pandemic, HIV is a surprisingly weak pathogen in vitro. The large difference between in vitro and in vivo infection rates suggests that cofactors absent in vitro but essential for the natural transmission of the virus may be responsible for this discrepancy. A recently identified peptide in human semen, PAP248-286, has emerged as a candidate for the missing cofactor as it dramatically enhances the infectivity of HIV by up to five orders of magnitude. The PAP248-286 peptide fragment has been shown to only induce its synergistic effect with HIV infection when in the form of amyloid fibers. Amyloid formation by PAP248-286 into the active SEVI form is a slow process during which it is susceptible to being degraded and inactivated. Therefore, initiators of this fibrillization process would be an indirect cause of the increase in viral infectivity. For this reason, we have searched for possible inhibitors and enhancers of SEVI amyloid formation including metals, lipids, other amyloids, and polyphenolic inhibitors. The effects of the metals are metal specific, with some enhancing kinetics, while others either inhibit or have little effect. High resolution structures of PAP248-286 and the green tea extract compound epigallocatechin gallate (EGCG) show binding to the monomer subunit through the lysine side-chains and inhibiting fiber growth, which could prove as an effective preventative measure for HIV infection. In contrast, amyloidogenic fibers produced by E. coli are seen to strongly enhance the kinetics of SEVI formation and HIV infectivity, indicating that bacterial infection could enhance the probability of HIV transmission. This phenomenon appears to be quite general and could be an important seeding mechanism for other amyloid proteins.1) Biophysical. Journal (2009), 97(9), 2474-2483.2) Biochemica et Biophysica Acta, Biomembranes (2011), 1808(4), 1161-1169.

Proteins misfolded into insoluble, fibrillar aggregates known as amyloid are a pathological feature of many common and devastating diseases. Amyloid formation is typically a slow process that can be strongly affected by extrinsic factors, among the most critical being the presence of a small amount of preformed seeds that serves to nucleate aggregation. Amyloid nucleation is often considered a highly specific process dependent on a high degree of similarity in both peptide sequence and fiber morphology. However, we show here that amyloid fibers known as curli that are produced in E. coli and related bacteria catalyze amyloid formation of a variety of dissimilar amyloidogenic peptides and proteins, including PAP248-286 (SEVI), insulin, and calcitonin. The preformed curli fibers appear to act as a nucleation site for amyloidogenic proteins and as such, can decrease the induction time, sometimes drastically, and induce the formation of fibers. In particular, cross-seeding of SEVI amyloid formation by curli was more effective than seeding the reaction with SEVI amyloid fibers obtained under a different reaction condition. The elongation rate of fiber formation is also increased for some (but not all) of the proteins tested, indicating curli can also increase in some circumstances the rate of addition of proteins to the ends of amyloid fibers. Curli and curli-like amyloid fibers are ubiquitous in mammalian hosts, in fact, the innate immune response common to almost all amyloids has been proposed to have evolved as a response to curli amyloid formation by E. Coli. Given that certain bacteria that express a curli-like protein colocalize with amyloid deposits in Alzheimer?s patients, the induction of amyloid formation by curli may be a factor of high clinical importance.