Abstract
Nuclear-encoded light-harvesting chlorophyll- and carotenoid-binding proteins (LHCPs) are imported into the chloroplast and transported across the stroma to thylakoid membrane assembly sites by the chloroplast signal recognition particle (CpSRP) pathway. The LHCP translocation defect (LTD) protein is essential for the delivery of imported LHCPs to the CpSRP pathway in Arabidopsis. However, the function of the LTD protein in Chlamydomonas reinhardtii has not been investigated. Here, we generated a C. reinhardtii ltd (Crltd) knockout mutant by using CRISPR-Cas9, a new target-specific knockout technology. The Crltd1 mutant showed a low chlorophyll content per cell with an unusual increase in appressed thylakoid membranes and enlarged cytosolic vacuoles. Profiling of thylakoid membrane proteins in the Crltd1 mutant showed a more severe reduction in the levels of photosystem I (PSI) core proteins and absence of functional LHCI compared with those of photosystem II, resulting in a much smaller PSI pool size and diminished chlorophyll antenna size. The lack of CrLTD did not prevent photoautotrophic growth of the cells. These results are substantially different from those for Arabidopsis ltd null mutant, indicating LTD function in LHCP delivery and PSI assembly may not be as stringent in C. reinhardtii as it is in higher plants.
Highlights
Photosynthesis relies on the linear coordinate function of two photosystems (PSI and PSII), which are light-absorbing complexes in the thylakoid membrane of photosynthetic organisms (Taiz and Zeiger, 2010)
The lack of CrLTD did not prevent photoautotrophic growth of the cells. These results are substantially different from those for Arabidopsis ltd null mutant, indicating LHCP translocation defect (LTD) function in light-harvesting chlorophyll- and carotenoid-binding protein (LHCP) delivery and photosystem I (PSI) assembly may not be as stringent in C. reinhardtii as it is in higher plants
Once an LHCP is inside the chloroplast, it is transported across the stroma to its final destination, the thylakoid membrane assembly sites, by the so-called chloroplast signal recognition particle (CpSRP) pathway (Henry, 2010; Richter et al, 2010)
Summary
Photosynthesis relies on the linear coordinate function of two photosystems (PSI and PSII), which are light-absorbing complexes in the thylakoid membrane of photosynthetic organisms (Taiz and Zeiger, 2010). Light-harvesting chlorophyll- and carotenoid-binding proteins (LHCPs) contain the photosynthetic pigments and function to absorb the energy of sunlight and transfer it to the reaction centers of the photosystems. LHCPs include LHCA and LHCB, Abbreviations: Chl, chlorophyll; CpSRP, chloroplast signal recognition particle; Crltd, C. reinhardtii ltd; indel, insertions and deletion; LHCI, light-harvesting complex I; LHCII, light-harvesting complex II; LHCP, light-harvesting chlorophyll- and carotenoid-binding protein; LTD, LHCP translocation defect; RNP, ribonucleoprotein; sgRNA, single guide RNA; TAP, Tris–acetate–phosphate; TBP, Tris–bicarbonate–phosphate. Once an LHCP is inside the chloroplast, it is transported across the stroma to its final destination, the thylakoid membrane assembly sites, by the so-called chloroplast signal recognition particle (CpSRP) pathway (Henry, 2010; Richter et al, 2010)
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