Proteinase-inhibitor activity and wound-inducible gene expression of the 22-kDa potato-tuber proteins

Using a proteinase-inhibition assay, we have demonstrated that the 22-kilodalton (kDa) potato (Solanum tuberosum L.) tuber proteins are strong inhibitors of serine proteinases. Two out of three purified proteins from the 22-kDa family of potato-tuber proteins were effective inhibitors of both trypsin and chymotrypsin, while the third, with a molecular mass (Mr) of approx. 24 kDa, inhibited only trypsin activity. Comparison of the amino-acid sequence of the putative reactive sites of several proteinase inhibitors with the deduced sequence of the 22-kDa protein showed that the 22-kDa protein contained sequences potentially possessing "doubleheaded" sites of inhibition, one against trypsin and another against chymotrypsin. The genes coding for the 22-kDa proteins were developmentally regulated in tubers and environmentally regulated in leaves. Wound induction of the genes coding for the 22-kDa potatotuber proteins was detected at the RNA level. In leaves, transcripts of the 22-kDa protein family were detected 6 h after wounding and were highest after 12 h in locally wounded leaves. The strongest induction occurred systemically in response to mechanical wounding in non-wounded leaves. Cross-hybridization of a cDNA, p34021, which codes for the 22-kDa tuber protein, with both proteinase-inhibitor I and II cDNAs and with a second family of 20-kDa potato-tuber cDNAs showed no cross-homology. Members of this second group of 20-kDa potato-tuber proteins also exhibited wound-induction in leaves at the RNA level.