Expression of a selenomethionyl derivative and preliminary crystallographic studies of human cystatin C

Human cystatin C, a protein with amyloidogenic properties and a potent inhi bitor of papain-like mammalian proteases, has been produced in its full-len gth form by recombinant techniques and crystallized in two polymorphic form s: cubic and tetragonal. A selenomethionyl derivative of the protein, obtai ned by Escherichia coli expression and with complete Met-->Se-Met substitut ion confirmed by mass spectrometry, amino-acid analysis and X-ray absorptio n spectra, was crystallized in the cubic form. A truncated variant of the p rotein, lacking ten N-terminal residues, has also been crystallized. The cr ystals of this variant are tetragonal and, like the two polymorphs of the f ull-length protein, contain multiple copies of the molecule in the asymmetr ic unit, suggesting oligomerization of the protein.