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In vitro study of stability and amyloid-fibril formation of two mutants of human stefin B (cystatin B) occurring in patients with EPM1

Sabina Rabzelj, Vito Turk and Eva erovnik

Department of Biochemistry and Molecular Biology, Joef Stefan Institute, 1000 Ljubljana, Slovenia

(RECEIVED May 26, 2005; FINAL REVISION July 7, 2005; ACCEPTED July 20, 2005)

Myoclonus epilepsy of type 1 (EPM1) is a rare monogenic progressive and degenerative epilepsy, also known under the name Unverricht-Lundborg disease. With the aim of comparing their behavior in vitro, wild-type (wt) human stefin B (cystatin B) and the G4R and the R68X mutants observed in EPM1 were expressed and isolated from the Escherichia coli lysate. The R68X mutant (Arg68Stop) is a peptide of 67 amino acids from the N terminus of stefin B. CD spectra have shown that the R68X peptide is not folded, in contrast to the G4R mutant, which folds like wild type. The wild type and the G4R mutant were unfolded by urea and by trifluoroethanol (TFE). It has been shown that both proteins have closely similar stability and that at pH 4.8, where a native-like intermediate was demonstrated, TFE induces unfolding intermediates prior to the major transition to the all--helical state. Kinetics of fibril formation were followed by Thioflavin T fluorescence while the accompanying changes of morphology were followed by the transmission electron microscopy (TEM). For the two folded proteins the optimal concentration of TFE producing extensive lag phases and high fibril yields was predenaturational, 9% (v/v). The unfolded R68X peptide, which is highly prone to aggregate, formed amyloid fibrils in aqueous solution and in predenaturing 3% TFE. The G4R mutant exhibited a much longer lag phase than the wild type, with the accumulation of prefibrillar aggregates. Implications for pathology in view of the higher toxicity of prefibrillar aggregates to cells are discussed.

Keywords: amyloid; cystatin B; epilepsy; folding intermediates; lag phase; prefibrillar aggregates; protein stability

Abbreviations: CD, circular dichroism • EPM1, progressive myoclonus epilepsy of Unverricht-Lundborg type • IPTG, isopropyl--D-thiogalactopyranoside • PCR, polymerase chain reaction • PEI, polyethyleneimine • SEC, size-exclusion chromatography • SDS-PAGE, sodium dodecyl sulfate-polyacrylamide gel electrophoresis • TEM, transmission electron microscopy • TFE, 2,2,2 Trifluorethanol • ThT, Thioflavin T • UV, ultraviolet.

Article published online ahead of print. Article and publication date are at http://www.proteinscience.org/cgi/doi/10.1110/ps.051609705.

Reprint requests to: Eva erovnik, Department of Biochemistry and Molecular Biology, Joef Stefan Institute, Jamova 39, 1000 Ljubljana, Slovenia; e-mail: eva.zerovnik{at}ijs.si; fax: +386-1-477-39-84.

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