Abstract
The carbon-concentrating mechanism (CCM) is essential to support photosynthesis under CO2-limiting conditions in aquatic photosynthetic organisms, including the green alga Chlamydomonas reinhardtii. The CCM is assumed to be comprised of inorganic carbon transport systems that, in conjunction with carbonic anhydrases, maintain high levels of CO2 around ribulose-1, 5-bisphosphate carboxylase/oxygenase in a specific compartment called the pyrenoid. A set of transcripts up-regulated during the induction of the CCM was identified previously and designated as low-CO2 (LC)-inducible genes. Although the functional importance of one of these LC-inducible genes, LciB, has been shown recently, the biochemical properties and detailed subcellular localization of its product LCIB remain to be elucidated. Here, using yeast two-hybrid, immunoprecipitation and mass spectrometry analyses we provide evidence to demonstrate that LCIB interacts with the LCIB homologous protein LCIC in yeast and in vivo. We also show that LCIB and LCIC are co-localized in the vicinity of the pyrenoid under LC conditions in the light, forming a hexamer complex of approximately 350 kDa, as estimated by gel filtration chromatography. LCIB localization around the pyrenoid was dependent on light illumination and LC conditions during active operation of the CCM. In contrast, in the dark or under high-CO2 conditions when the CCM was inactive, LCIB immediately diffused away from the pyrenoid. Based on these observations, we discuss possible functions of the LCIB-LCIC complex in the CCM.
Highlights
In nature, the ambient carbon dioxide (CO2) concentration of 0.038% [low-CO2 (LC)] is one of the major limiting factors for photosynthesis
To evaluate the expression of the respective gene products in the photosynthetically wild-type (WT) 5D strain, which was used throughout this study, crude protein extracts were prepared from 5D cells grown under HC and LC conditions, electrophoresed and probed with antibodies against LCIB or LCIC synthetic polypeptides (ANAPTMESPEPVHP and PTMDSPELANMN, respectively) (Fig. 1A)
The LCIB and LCIC mature polypeptides were slightly accumulated under HC conditions, and their protein levels were induced after shifting to LC conditions, reaching saturation within 6 h
Summary
The ambient carbon dioxide (CO2) concentration of 0.038% [low-CO2 (LC)] is one of the major limiting factors for photosynthesis. To adapt to LC environments, aquatic photosynthetic organisms have developed a carbon-concentrating mechanism (CCM) to optimize their photosynthetic rate. Aquatic photosynthesis under LC conditions shows characteristics similar to those seen in C4 photosynthesis, the microalgal CCM and C4 photosynthetic mechanisms are very different. The CCM, uptake systems for dissolved inorganic carbon (Ci; CO2 and bicarbonate) increases the CO2 concentration at the site of ribulose-1, 5-bisphosphate carboxylase/ oxygenase (Rubisco) (Badger et al 1980, Fukuzawa et al 2001, Giordano et al 2005, Spalding 2008). At least five types of transport systems involved in Ci uptake have been characterized in cyanobacteria, including three bicarbonate transporters and two CO2 uptake systems (Price et al 2008).
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