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Functional analysis of plant metacaspases type II

Final Activity Report Summary - PLANT METACASPASES (Functional analysis of plant metacaspases type II)

Metacaspases are genuine cysteine proteases that autoprocess in a way similar to caspases for which no functional role has been described yet. They are suggested to be the ancestors of metazoan caspases and there is some indication that they can be involved in apoptotic cell death in yeast. In plant after some first indications of their possible role in programmed cell death their function is still unclear. In the model plant Arabidopsis thaliana 9 genes coding for metacaspases are present.

In the attempt to understand the biological function of the Arabidopsis thaliana Metacaspase9 (AtMC9) we isolated and analysed putative interactors and substrates. At first we have concentrated our studies on proteins that interact with AtMC9 in a yeast 2 hybrid screen and that can be cleaved by the recombinant protein. We identified suicide substrate inhibitor named Serpin that can inhibit AtMC9. This is the first reported plant protease that can be inhibited by a plant Serpin.

From an in vivo screening of potential substrate for AtMC9 we identified a chaperon of the HSP100 family that is the regulatory subunits of Clp protease named ClpC (caseinolytic protease proteins). Hsp100 proteins are molecular chaperones involved in a wide variety of essential metabolic processes that can be found both in prokaryote and eukaryote. Cleavage of ClpC by metacaspase9 seems to occur physiologically during the first days of germination and during leaves senescing.

Moreover, we assessed a role for Nitric Oxide in regulating the proteolytic activity of Arabidopsis AtMC9. We found that AtMC9 zymogens are S-nitrosylated at their active site cysteine residues in planta, and that this post-translational modification suppresses both AtMC9 autoprocessing and proteolytic activity. In addition we identified a second catalytic cysteine which is not present in caspase proteins and is not susceptible to S-nitrosylation. We show that this second cysteine is not S-nitrosylated and can replace the S-nitrosylated cysteine residue within the catalytic centre of the processed AtMC9. Together, these data demonstrate that S-nitrosylation plays a central role in regulation plant metacaspase function and suggest a new link between changes in the redox state of the cell and the activation of signalling pathways.