The aggregation of proteins into amyloid fibrils and their deposition into plaques and intracellular inclusions is the hallmark of amyloid disease. The accumulation and deposition of amyloid fibrils, collectively known as amyloidosis, is associated with many pathological conditions such as A disease P , type II diabetes, and dialysis related amyloidosis.However, elucidation of the atomic structure of amyloid fibrils formed from their intact protein precursors and how fibril formation relates to disease has remained elusive. Recent advances in structural biology techniques, including cryo-electron microscopy (cryo-EM) and solid state NMR (ssNMR), have finally broken this impasse. The first near-atomic resolution structures of amyloid fibrils formed in vitro, seeded from plaque material, and analysed directly ex vivo are now available.The results reveal cross- structures which are far more intricate than anticipated. Here, we describe these structures, highlighting their similarities and differences. We also discuss how amyloid structure may affect the ability of fibrils to spread to different sites in a prion-like manner, along with their roles in disease. These molecular insights will aid in understanding the development and spread of amyloid diseases and are inspiring new strategies for therapeutic intervention.

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A Shiga-like toxin type II variant (SLT-IIv) is produced by strains of Escherichia coli responsible for edema disease of swine and is antigenically related to Shiga-like toxin type II (SLT-II) of enterohemorrhagic E. coli. However, SLT-IIv is only active against Vero cells, whereas SLT-ll is active against both Vero and HeLa cells.The structural genes for SLT-llv were cloned from E. coli S1191, and the nucleotide sequence was determined and compared with those of other members of the Shiga toxin family. The A subunit genes for SLT-IIv and SLT-II were highly homologous (94%), whereas the B subunit genes were less homologous (79%). The SLT-IIv genes were more distantly related (55 to 60% overall homology) to the genes for Shiga toxin of Shigella dysenteriae type 1 and the nearly identical Shiga-like toxin type I (SLT-I) of enterohemorrhagic E. coli. (These toxins are referred to together as Shiga toxin/SLT-I.) The A subunit of SLT-IIv, like those of other members of this toxin family, had regions of homology with the plant lectin ricin. SLT-IIv did not bind to galactose-al-4-galactose coijugated to bovine serum albumin, which is an analog of the eucaryotic cell receptor for Shiga toxin/SLT-I and SLT-ll. These findings support the hypothesis that SLT-IIv binds to a different cellular receptor than do other members of the Shiga toxin family but has a similar mode of intracellular action, The organization of the SLT-llv operon was similar to that of other members of the Shiga toxin family. Iron did not suppress SLT-IIv or SLT-II production, in contrast with its effect on Shiga toxinlSLT-I. Therefore, the regulation of synthesis of SLT-IIv and SLT-II differs from that of Shiga toxhi/SLT-I.Some Escherichia coli strains produce cytotoxins that are related to the Shiga toxin produced by Shigella dysenteriae type 1. These Shiga-like toxins (SLTs), which are also called verotoxins (19) Letter, Lancet i:702, 1983). Two antigenically distinct types of SLTs, SLT-I and SLT-II, that cause hemorrhagic colitis and/ or the hemolytic-uremic syndrome have been isolated from E. coli (39; S. M. Scotland, H. R. Smith, G. A. Willshaw, and B. Rowe, Letter, Lancet ii:216, 1983). SLT-I but not SLT-II is neutralized by polyclonal antisera to Shiga toxin (39). Shiga toxin, SLT-I, SLT-II, and the plant toxin ricin have the same mechanism of action (10, 11). These toxins are N-glycosidases that cleave a specific adenine residue in the 28S subunit of eucaryotic rRNA which in turn causes protein synthesis to cease. The A subunit of each of these toxins is responsible for the N-glycosidase activity. Hovde et al. recently demonstrated that amino acid 167 (glutamic acid) in the A subunit of SLT-I is critical for the enzymatic activity of the toxin (13). The A subunit of Shiga toxin, SLT-I, and presumably SLT-II is noncovalently linked to multiple copies of the B subunit (9). The eucaryotic receptor to which the B subunits of Shiga toxin and SLT-I bind is a galactose-al-4-galactose-containing glycolipid designated Gb3 (16,(21)(22)(23) STF-3, 19...

The structural genes for Shiga toxin, designated stx A and stx B, were cloned from Shigella dysenteriae type 1 3818T, and a nucleotide sequence analysis was performed. Both stx A and stx B were present on a single transcriptional unit, with stx A preceding stx B. The molecular weight calculated for the processed A subunit was 32,225, while the molecular weight of the processed B subunit was 7,691. Comparison of the nucleotide sequences for Shiga toxin and Shiga-like toxin I (SLT-I) from Escherichia coli revealed that the genes for Shiga toxin and SLT-I were greater than 99% homologous; three nucleotide changes were detected in three separate codons of the A subunits. Only one of these codon differences resulted in a change in the amino acid sequence: a threonine in Shiga toxin at position 45 of the A subunit compared with a serine in the corresponding position in SLT-I. Furthermore, Shiga toxin and SLT-I had identical signal peptides for the A and B subunits, as well as identical ribosome-binding sites, a putative promoter, and iron-regulated operator sequences. These findings indicate that Shiga and SLT-I are essentially the same toxin. Southern hybridization studies with total cellular DNA from several Shigella strains and internal toxin probes for SLT-I and its antigenic variant SLT-II showed that a single fragment in S. dysenteriae type 1 hybridized strongly with the internal SLT-I probe. Fragments with weaker homology to the SLT-I probe were detected in S. flexneri type 2a but no other shigellae. No homology between the Shiga-like toxin II (SLT-II) probe and any of the Shigella DNAs was detected. Whereas SLT-I and SLT-II are phage encoded, no phage could be induced from S. dysenteriae type 1 or other Shigella spp. tested. These results suggest that the Shiga (SLT-I) toxin genes responsible for high toxin production are present in a single copy in S. dysenteriae type 1 but not in other shigellae. The findings further suggest that SLT-II genes are absent in shigellae, as are toxin-converting phages.Shiga toxin is a cell-associated protein toxin composed of one copy of an A subunit (molecular weight estimated as 32,000) and five copies of a B subunit (molecular weight estimated as 7,700) (8, 38). The toxin inhibits protein synthesis in eucaryotic cells by cleaving the N-glycosidic bond at adenine 4324 in 28S rRNA (Y. Endo, K. Tsurugi, T. Yutsudo, Y. Takeda, T. Ogasawara, and K. Igarashi, Eur. J. Biochem., in press). The mode of action of Shiga toxin is therefore identical to that of the plant toxin ricin (10). The biological and biochemical properties of Shiga toxin have recently been reviewed by O'Brien and Holmes (27).Shigella species other than Shigella dysenteriae type 1 and certain strains of Escherichia coli, Salmonella typhimurium, Vibrio cholerae, and Campylobacter jejuni produce low levels of a cytotoxin(s) that is neutralizable by antibodies against purified Shiga toxin from S. dysenteriae type 1 (30, 32; A. D. O'Brien, M. E. Chen, R. K. Holmes, J. Kaper, and M. Levine, Letter, Lancet, i:77-78, 1984;...