Expression of a selenomethionyl derivative and preliminary crystallographic studies of human cystatin C
Human cystatin C, a protein with amyloidogenic properties and a potent inhibitor of papain‐like mammalian proteases, has been produced in its full‐length form by recombinant techniques and crystallized in two polymorphic forms: cubic and tetragonal. A selenomethionyl derivative of the protein, obtained by Escherichia coli expression and with complete Met→Se‐Met substitution confirmed by mass spectrometry, amino‐acid analysis and X‐ray absorption spectra, was crystallized in the cubic form. A truncated variant of the protein, lacking ten N‐terminal residues, has also been crystallized. The crystals of this variant are tetragonal and, like the two polymorphs of the full‐length protein, contain multiple copies of the molecule in the asymmetric unit, suggesting oligomerization of the protein.