Identification And Functional Analysis Of A Novel NADPH Dehydrogenase Subcomplex In Cyanobacteria

Two decades ago, the fifth cyanobacterial photosynthetic membrane protein complex, NADPH dehydrogenase (NDH-1), was discovered in the thylakoid membrane, and is essential to CO₂ uptake, cyclic electron transport around photosystem I and cellular respiration. So far, the NDH-1 at least consists of 17 subunits, i.e., NdhA to NdhQ; among of them, mutation of NdhB subunit resulted in lacking of all of physiological functions of NDH-1, but the cyanobacterial cells have still the ability to normally grow under high CO₂ conditions. In contrast, the deletion of NdhH subunit led to the cell death under all conditions of growth. This dissertation attempted to investigate the controversial scientific question.In 2004, two types of functionally distinct NDH-1 complexes, NDH-1L (molecular weight approximately 460 kDa) and NDH-1MS (molecular weight approximately 490 kDa), were identified in cyanobacterial cells by using proteomic methods. The NDH-1L complex involves in cellular respiration while NDH-1MS is essential for CO₂ uptake; further, both NDH-1 complexes are necessary to cyclic electron transport around photosystem I. However, these two functional complexes of NDH-1 are absent in M55 cells. Recently, we identified a new NDH-1 subcomplex (molecular weight approximately 160 kDa) in M55 strain by using biochemical methods, and designated it Subcomplex A. The results with immunoblotting indicated that this subcomplex A at least includes four Ndh subunits, i.e., NdhH, I, K and M. Further, this subcomplex A is essential for the cyclic electron transport around photosystem I as assessed by the chlorophyll fluorescence parameters and supported by the changes in protein levels of Ndh subunits. In conclusion, we conclude that sub- complex A exists in M55 cells, and essential for cells to survive under high CO₂ conditions. Therefore, the discovery of the novel subcomplex will help in clarifying the previous controversial scientific question, and providing a new insight into understanding of the physiological role of cyanobacterial NDH-1 complexes.