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Death proteases: alive and kicking

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Two recent discoveries significantly add to our understanding of plant programmed cell death (PCD). Hatsugai et al. showed that cell death is dependent on proper proteasome functioning. Sundström et al. showed that the in vivo substrate of a type II metacaspase is associated with cell viability. Both findings are major breakthroughs within the plant PCD field and highlight that the plant cell death machinery apparently employs a wide range of structurally unrelated proteases that, surprisingly, show a caspase-like preference for specific (evolutionarily conserved) substrates.

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Structural similarities between PCD in animal and plant cells

Programmed cell death (PCD) in animal cells is often executed by caspases. Caspases belong to a class of specific cysteine proteases that show a high degree of specificity, with an absolute requirement for cleavage adjacent to an aspartate residue and a recognition sequence comprised of at least four amino acids N-terminal to this cleavage site. In animal cells, a number of morphological and biochemical features accompanying cell death (e.g. chromatin condensation and nuclear and DNA

The plant proteasome confers caspase-like activity

The 26S proteasome is responsible for the breakdown of the majority of proteins and consists of the 20S proteasome and one or two 19S regulatory complexes. The 20S proteasome is a hollow cylinder composed of four stacked rings; the two inner beta rings are identical in subunit composition and each beta ring contains three different proteolytic sites designated β1, β2 and β5. Previously it was shown that animal and yeast (Saccharomyces cerevisiae) proteasomes show caspase-like activity. These

Type II metacaspase cleaves a conserved caspase3 substrate

Metacaspases (MCAs) are cysteine proteases that show structural similarity to caspases (specifically the caspase hemoglobinase fold) but instead of cleavage adjacent to Asp, MCAs show a proteolytic activity against arginine and lysine in the P1 position. Arabidopsis has nine MCA genes; three of type I and six of type II. Type I MCAs have an N-terminal extension reminiscent of the prodomain of initiator caspases. This prodomain contains two putative CXXC-type zinc finger structures similar to

Death protease diversity and substrate conservation?

The cell death machinery in plants seems more divergent than in animal cells, where cell death is often executed through the coordinated action of a small family of structurally related cysteine proteases (caspases). Plant cells apparently employ a wide range of structurally unrelated proteases that, surprisingly, show a caspase-like preference for specific (conserved) sequences in their substrates. Currently a number of death proteases have been identified. For most of these proteases the cell

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