Immunophilin CYN28 is required for accumulation of photosystem II and thylakoid FtsH protease in Chlamydomonas

The Role of FtsH Complexes in the Response to Abiotic Stress in Cyanobacteria

FtsH proteases (FtsHs) belong to intramembrane ATP-dependent metalloproteases which are widely distributed in eubacteria, mitochondria and chloroplasts. The best-studied roles of FtsH in Escherichia coli include quality control of membrane proteins, regulation of response to heat shock, superoxide stress and viral infection, and control of lipopolysaccharide biosynthesis. While heterotrophic bacteria mostly contain a single indispensable FtsH complex, photosynthetic cyanobacteria usually contain three FtsH complexes: two heterocomplexes and one homocomplex. The essential cytoplasmic FtsH1/3 most probably fulfills a role similar to other bacterial FtsHs, whereas the thylakoid FtsH2/3 heterocomplex and FtsH4 homocomplex appear to maintain the photosynthetic apparatus of cyanobacteria and optimize its functionality. Moreover, recent studies suggest the involvement of all FtsH proteases in a complex response to nutrient stresses. In this review, we aim to comprehensively evaluate the functions of the cyanobacterial FtsHs specifically under stress conditions with emphasis on nutrient deficiency and high irradiance. We also point to various unresolved issues concerning FtsH functions, which deserve further attention.

Immunophilin FKB20-2 participates in oligomerization of Photosystem I in Chlamydomonas

PSI is a sophisticated photosynthesis protein complex that fuels the light reaction of photosynthesis in algae and vascular plants. While the structure and function of PSI have been studied extensively, the dynamic regulation on PSI oligomerization and high light response is less understood. In this work, we characterized a high light-responsive immunophilin gene FKB20-2 (FK506-binding protein 20-2) required for PSI oligomerization and high light tolerance in Chlamydomonas (Chlamydomonas reinhardtii). Biochemical assays and 77-K fluorescence measurement showed that loss of FKB20-2 led to the reduced accumulation of PSI core subunits and abnormal oligomerization of PSI complexes and, particularly, reduced PSI intermediate complexes in fkb20-2. It is noteworthy that the abnormal PSI oligomerization was observed in fkb20-2 even under dark and dim light growth conditions. Coimmunoprecipitation, MS, and yeast 2-hybrid assay revealed that FKB20-2 directly interacted with the low molecular weight PSI subunit PsaG, which might be involved in the dynamic regulation of PSI-light-harvesting complex I supercomplexes. Moreover, abnormal PSI oligomerization caused accelerated photodamage to PSII in fkb20-2 under high light stress. Together, we demonstrated that immunophilin FKB20-2 affects PSI oligomerization probably by interacting with PsaG and plays pivotal roles during Chlamydomonas tolerance to high light.

Advances in light system engineering across the phototrophic spectrum

Current work in photosynthetic engineering is progressing along the lines of cyanobacterial, microalgal, and plant research. These are interconnected through the fundamental mechanisms of photosynthesis and advances in one field can often be leveraged to improve another. It is worthwhile for researchers specializing in one or more of these systems to be aware of the work being done across the entire research space as parallel advances of techniques and experimental approaches can often be applied across the field of photosynthesis research. This review focuses on research published in recent years related to the light reactions of photosynthesis in cyanobacteria, eukaryotic algae, and plants. Highlighted are attempts to improve photosynthetic efficiency, and subsequent biomass production. Also discussed are studies on cross-field heterologous expression, and related work on augmented and novel light capture systems. This is reviewed in the context of translatability in research across diverse photosynthetic organisms.

Regulation of chloroplast protein degradation

Chloroplasts are unique organelles that not only provide sites for photosynthesis and many metabolic processes, but also are sensitive to various environmental stresses. Chloroplast proteins are encoded by genes from both nuclear and chloroplast genomes. During chloroplast development and responses to stresses, the robust protein quality control systems are essential for regulation of protein homeostasis and the integrity of chloroplast proteome. In this review, we summarize the regulatory mechanisms of chloroplast protein degradation refer to protease system, ubiquitin-proteasome system, and the chloroplast autophagy. These mechanisms symbiotically play a vital role in chloroplast development and photosynthesis under both normal or stress conditions.