In animals, the main actors of apoptosis are proteases termed caspases. Unexpectedly, genes coding for structurally homologous proteins were recently identified also in prokaryotes and are especially abundant in Cyanobacteria. We have been the first to biochemically characterize these proteins and have due to their catalytic properties and evolutionary early emergence termed them orthocaspases [Klemencic et al., 2015]. As opposed to higher plants or animals, some algae and majority of cyanobacteria contain also proteins, that instead of the catalytic domain contain proteolytically inactive variants, they are therefore pseudo-enzymes. We recently reported their distribution in prokaryotic organisms [Klemencic et al., 2019]. To study the function of these proteins in vitro as well as in vivo, we are constructing genetically modified variants of orthocaspases in our two model cyanobacteria: Microcystis aeruginosa PCC 7806, which contains six different orthocaspase genes and Synechocystis sp. PCC 6803, containing only one pseudo-orthocaspase.
All up to now characterized unicellular eukaryotic organisms contain a different variant of caspase homologues than the prokaryotes do; these are termed metacaspases. We aim to understand how the differences in their structures govern their function. We so far know that activity of all metacaspases depends on the presence of calcium ions. We have recently shown [Klemencic and Funk, 2018] that it is the aspartic acid residues in the C-terminal of the protease that are involved in calcium ion coordination and are thus required for proteolytic activity of these enzymes. Our two algal model organisms for studying metacaspases as the model alga, Chlamydomonas reinhardtii, and a secondary endosymbiote, Guillardia theta.
