Specificity of Protein Turnover in Tomato Leaves

Detached tomato leaves, supplied with the proteinase inhibitor inducing factor (PIIF) and incubated with water under constant light, exhibited a specificity of intracellular protein turnover directed toward the selective accumulation of heat-stable proteins having disulfide cross-linkages. Approximately 70% of the accumulated proteins could be accounted for in two proteinase.inhibitors rich in disulfide links. The accumulation of proteins containing disulfides was accompanied by a net loss in total leaf protein, mainly of heat-precipitable proteins having free sulfhydryl residues. Relative rates of synthesis of -S-Sproteins and -SH proteins were assessed by comparing rates of incorporation of isotope into the inhibitor proteins and noninhibitor leaf proteins. Although the inhibitors represented about 12% of total leaf protein after 71 h of induction, only about 2% of total protein synthesis was directed toward inhibitor synthesis during incubation of induced leaves. The marked stability of inhibitors, and other disulfide proteins against degradation in uiuo, appeared to be a major factor providing for their selective accumulation. It was concluded that the state of oxidation of protein-bound half-cystine residues may be a principle parameter influencing the susceptibility of leaf proteins to degradation in uiuo. In response to wounding, tomato plants accumulate large quantities of two proteinaceous inhibitors, of chymotrypsin, called Inhibitor I and Inhibitor II (l-3). Inhibitor accumulation is directed by a hormone-like substance, called the proteinase inhibitor inducing factor (PIIF), which is released from wound sites and translocated throughout the plant’s vascular system. Since leaf damage, caused by insects (1) and fungal pathogens, 1