*2.10. Thimet Oligopeptidases* + *TPPII Exopeptidase—Proteolysis*

In *A. thaliana*, there are three thimet oligopeptidases (TOP, zinc-dependent metalloendopeptidases) [90]. Two of them, TOP1 and TOP2, bind SA [49]. SA inhibited peptidase activities of TOP1 and, to a lesser extent, of TOP2, in vitro. For TOP1, kinetics indicated a non-competitive mechanism. SA treatment also inhibited the bulk peptidase activity in plant extracts as measured by the release of a fluorescent peptide marker. TOP1 contains a signal peptide and TOP1–GFP was found to be localized in chloroplasts. The inhibitory effect of SA on a truncated form of TOP1, lacking 110 N–terminal residues spanning the signal peptide, was much weaker. This suggests that SA could selectively affect TOP1 activity based on its localization. Based on mutant studies, both TOP1 and TOP2 were required for plant response to either Pst *avrRpt2* or Pst *avrRps4*. However, when tested with Pst *avrRpm1*, Pst *avrPphB* or *Pseudomonas syringae* pv. *maculicola* (Psm), no differences to WT plants were observed. Moreover, the assessed level of programmed cell death was actually lower in *top1-3* mutants inoculated with Pst avrRpt2 compared to WT [49].

TOP1 and TOP2 were found to produce homo- and heterodimers. The formation of TOP2-TOP2 and TOP1-TOP2 dimers were diminished by the addition of SA in isolated *A. thaliana* protoplasts. Authors suggested that the effect of SA could be due to redox changes since the effect of dithiothreitol, a strong reductant, led to a strong shift towards the presence of TOP1 and TOP2 monomeric forms in vitro [91]. The functional role of TOP dimers is unclear.

Another enzyme implicated in proteolysis that binds SA is tripeptidyl peptidase II [50].

#### *2.11. MORC Proteins—Epigenetic Regulation*

Microrchidia (MORC) proteins comprise a group of peculiar DNA-binding enzymes with ATPase, endonuclease and topoisomerase activities. These proteins can be potentially involved in epigenetic gene silencing [92]. In tomato, SlMORC1 binds SA as demonstrated by SPR analysis [93]. This interaction resulted in altered activities of SlMORC1 in vitro: SA suppressed ATPase and decatenation activities but not the DNA relaxation activity of SlMORC1.
