Repair of the thylakoid photosystem damaged by intense light

Removal of the D1 protein during "light stress".

Photosynthesis represents one of the most important chemical reactions on Earth. During this process solar energy is used to drive electrons along the photosynthetic electron transfer chain, thereby generating a pH gradient across the thylakoid membrane that is required for ATP synthesis. However, under intense light conditions, the same life-giving solar energy can also severely damage the photosynthetic machinery, which leads to photoinhibition. The protein that is most prone to light-induced damage is the reaction center protein D1 of the photosystem II (PSII). To protect against such damage plants have evolved a protective mechanism, which degrades photodamaged D1 and replaces it with de-novo synthesized protein. Proteolysis of photodamaged D1 is a highly coordinated process carried out by the chloroplast proteases Deg1, Deg5/8 and FtsH. To better understand how Deg1 initiates the corresponding proteolytic cascade, we performed a comprehensive structure-function analysis of Deg1 from Arabidopsis thaliana.

Our findings: 2010 (Regulation of Deg1 by pH-dependent oligomer re-assembly)

2010 Deg1 can sense light-stress conditions and prevent photoinhibition

Overall structure of Deg1 highlighting the pH-sensing His that controls trimer formation.
Light-stress induced activation of Deg1, as sensed by local pH changes.

We determined the crystal structure of the Deg1 hexamer at 2.5 Å resolution. The hexameric cage of Deg1 consists of two trimeric rings stacked on top of each other with the active sites sequestered inside the cage. Because the proteolytic active sites are only accessible from the interior, substrate molecules must enter the Deg1 hexamer through three pores, which restrict access to unfolded proteins and extended polypeptide loop structures. Moreover, hexamer formation immobilizes the PDZ domains in proper position to interact with the protease domain and to stabilize its active conformation. Biochemical analyses revealed that Deg1 converts from the resting monomer into the functional hexamer at slightly acidic pH. The change in pH is sensed by His244. Once His244 is protonated, it properly positions the N-terminal helix of the protease for trimer- and hexamerization. The identified pH-dependent switch ensures that the Deg1 protease is selectively activated under light stress conditions, at which acidification of the thylakoid lumen occurs and photosynthetic proteins are damaged. Once activated, the Deg1 hexamer can initiate degradation of the oxidized D1 protein, to guarantee efficient repair of PSII. The activation mechanism of Deg1 illustrates how PDZ-proteases can function as pH sensors in stress-response pathways.