Thermoprotection by a cell membrane-localized metacaspase in a green alga
Caspases are restricted to animals, while other organisms, including plants possess metacaspases (MCAs), a more ancient and broader class of structurally-related yet biochemically distinct proteases. Our current understanding of plant MCAs is derived from studies in streptophytes, and mostly in Arabidopsis expressing nine MCAs with partly redundant activities. In contrast to streptophytes, most chlorophytes contain only one or two hitherto uncharacterized MCAs, providing an excellent platform for MCA research. Here we investigate CrMCA-II, a single type II MCA from a model chlorophyte Chlamydomonas reinhardtii. Surprisingly, unlike other studied MCAs and similar to caspases, CrMCA-II dimerizes both in vitro and in vivo. Furthermore, activation of CrMCA-II in vivo correlates with the dimerization. Most of CrMCA-II in the cell is present as a zymogen attached to the plasma membrane (PM). Deletion of CrMCA-II by CRISPR/Cas9 compromises thermotolerance leading to increased cell death under heat stress. Adding back either wild-type or catalytically dead CrMCA-II restores thermoprotection, suggesting that its proteolytic activity is dispensable for this effect. Finally, we link the non-proteolytic role of CrMCA-II in thermotolerance to the ability to modulate PM fluidity. Our study reveals an ancient, MCA-dependent thermotolerance mechanism retained by Chlamydomonas and probably lost during the evolution of multicellularity.