Expression in Escherichia coli of cysteine protease inhibitors from cowpea (Vigna unguiculata): The crystal structure of a single-domain cystatin gives insights on its thermal and pH stability.

Two cysteine proteinase inhibitors from cowpea, VuCys1 and VuCys2, were produced in E. coli ArcticExpress (DE3). The recombinant products strongly inhibited papain and chymopapain as well as the midgut proteases from Callosobruchus maculatus larvae, a bruchid that uses cysteine proteases as major digestive enzymes. Heat treatment at 100°C for up to 60min or incubation at various pH values caused little reduction in the papain inhibitory activity of both inhibitors. Moreover, minor conformational variations, as probed by circular dichroism spectroscopy, were observed after VuCys1 and VuCys2 were subjected to these treatments. The crystal structure of VuCys1 was determined at a resolution of 1.95Å, revealing a domain-swapped dimer in the asymmetric unit. However, the two lobes of the domain-swapped dimer are positioned closer to each other in VuCys1 in comparison to other similar cystatin structures. Moreover, some polar residues from opposite lobes recruit water molecules, forming a hydrogen bond network that mediates contacts between the lobes, thus generating an extended open interface. Due to the closer distance between the lobes, a small hydrophobic core is also formed, further stabilizing the folded domain-swapped dimer. These structural features might account for the extraordinary thermal and pH stability of VuCys1.

Expression of the Canavalia brasiliensis lectin (ConBr) in tobacco plants.

Tobacco plants were transformed with gene constructs encoding prepro-ConBr (Canavalia brasiliensis lectin). Transgenic plants confirmed by PCR expressed the recombinant protein as revealed by Western blot. However, the apparent molecular mass of the recombinant polypeptide (ca. 34 kDa) was higher than the native lectin (about 30 kDa), showing that further proteolytic processing of pro-ConBr was not detected.