Structural adaptation of the plant protease Deg1 to repair photosystem II during light exposure

Deg1 is an HtrA protease that participates in the turnover of the photosynthetic proteins in chloroplasts. Now the crystal structure of Deg1, along with functional work, reveals that Deg1 oligomerization and activation occur in response to acidic pH, which should be encountered in the thylakoid lumen when exposed to light. Deg1 is a chloroplastic protease involved in maintaining the photosynthetic machinery. Structural and biochemical analyses reveal that the inactive Deg1 monomer is transformed into the proteolytically active hexamer at acidic pH. The change in pH is sensed by His244, which upon protonation, repositions a specific helix to trigger oligomerization. This system ensures selective activation of Deg1 during daylight, when acidification of the thylakoid lumen occurs and photosynthetic proteins are damaged.