ABSTRACT
Fluorescent derivatives of the ß-amyloid peptides (Aß) are valuable tools for studying the interactions of Aß with cells. Facile access to labeled expressed Aß offers the promise of Aß with greater sequence and stereochemical integrity, without impurities from amino acid deletion and epimerization. Here, we report methods for the expression of Aß42 with an N-terminal cysteine residue, Aß(C1-42), and its conjugation to generate Aß42 bearing fluorophores or biotin. The methods rely on the hitherto unrecognized observation that expression of the Aß(MC1-42) gene yields the Aß(C1-42) peptide, because the N-terminal methionine is endogenously excised by Escherichia coli. Conjugation of Aß(C1-42) with maleimide-functionalized fluorophores or biotin affords the N-terminally labeled Aß42. The expression affords â¼14 mg of N-terminal cysteine Aß from 1 L of bacterial culture. Subsequent conjugation affords â¼3 mg of labeled Aß from 1 L of bacterial culture with minimal cost for labeling reagents. High-performance liquid chromatography analysis indicates the N-terminal cysteine Aß to be >97% pure and labeled Aß peptides to be 94-97% pure. Biophysical studies show that the labeled Aß peptides behave like unlabeled Aß and suggest that labeling of the N-terminus does not substantially alter the properties of the Aß. We further demonstrate applications of the fluorophore-labeled Aß peptides by using fluorescence microscopy to visualize their interactions with mammalian cells and bacteria. We anticipate that these methods will provide researchers convenient access to useful N-terminally labeled Aß, as well as Aß with an N-terminal cysteine that enables further functionalization.
