An osmotin-like cryoprotective protein from the bittersweet nightshade Solanum dulcamara

Cold acclimation in plants is a polygenic phenomenon involving increased expression of several genes. The gene products participate either directly or indirectly towards increasing cold tolerance. Evidence of proteins having a direct effect on cold tolerance is emerging but limited. With isolated protoplasts from warm-grown kale (Brassica oleracea) as a model system, we tested protein fractions from winter bittersweet nightshade, Solanum dulcamara, stems for the presence of proteins that have a cryoprotective effect. Purification of one such fraction resulted in isolation of a 25 kDa protein. N-terminal Edman degradation amino acid sequence analysis showed that it has high homology to osmotin and osmotin-like proteins. When added to warm-grown protoplasts, it increased the cryosurvival of frozen-thawed protoplasts by 24% over untreated or BSA-treated controls at -8 degreesC. A cDNA library which was made in November from stems and leaves of S. dulcamara was successfully screened for the corresponding cDNA clone. The deduced amino acid sequence indicated that the protein consists of 206 amino acid residues including a N-terminal signal sequence and a putative C-terminal propeptide. The mature protein, without the N-terminal signal sequence, was expressed in Escherichia coli. The partially purified protein in the supernatant fraction of the culture medium had cryoprotective activity.