Double mutation cpSRP43^–/cpSRP54^–is necessary to abolish the cpSRP pathway required for thylakoid targeting of the light‐harvesting chlorophyll proteins

Abstract

SummaryBiochemical and genetic studies have established that the light-harvesting chlorophyll proteins (LHCPs) of the photosystems use the cpSRP (chloroplast signal recognition particle) pathway for their targeting to thylakoids. Previous analyses of single cpSRP mutants, chaos and ffc, de®cient in cpSRP43 and cpSRP54, respectively, have revealed that half of the LHCPs are still integrated into the thylakoid membranes. Surprisingly, the effects of both mutations are additive in the double mutant ffc/chaos described here. This mutant has pale yellow leaves at all stages of growth and drastically reduced levels of all the LHCPs except Lhcb 4. Although the chloroplasts have a normal shape, the thylakoid structure is affected by the mutation, probably as a consequence of reduction of all the LHCPs. ELIPs (early lightinducible proteins), nuclear-encoded proteins related to the LHCP family and inducible by light stress, were also drastically reduced in the double mutant. However, proteins targeted by other chloroplastic targeting pathways (DpH, Sec and spontaneous pathways) accumulated to similar levels in the wild-type and the double mutant. Therefore, the near total loss of LHCPs and ELIPs in the double mutant suggests that cpSRP is the predominant, if not exclusive, targeting pathway for these proteins. Phenotypic analysis of the double mutant, compared to the single mutants, suggests that the cpSRP subunits cpSRP43 and cpSRP54 contribute to antenna targeting in an independent but additive way.

LTD is a protein required for sorting light-harvesting chlorophyll-binding proteins to the chloroplast SRP pathway

“…The PSI and PSII core subunits PsaA/B, D2 and CP47 were reduced to about 50% of wild-type levels, whereas the levels of OE33, Cytf of Cytb 6 /f and the β-subunit of ATP synthase were unchanged compared with wild type. A previous report suggested that depletion of LHCP might result in impaired stability of photosynthetic proteins 29 . Pulse labelling for 20 min followed by a chase with unlabelled methionine in the presence of cycloheximide, which blocks the synthesis of nuclear-encoded proteins, showed that the turnover rates of the PSI and PSII subunits (PsaA/B, CP47, CP43, D1 and D2) were higher in ltd than in wild-type plants ( Supplementary Fig.…”

“…Input, 4% of the prey protein. pigment deficiency phenotypes 16,21,23,28,29,34 . The ltd mutant did not develop any fertile flowers (Fig.…”

“…The T-DNA insertion lines ltd (SALK_126967C), cpftsY (SALK_049077) 34 and ffc mutant (CS850421) were obtained from the Arabidopsis Biological Resources Centers, and the homozygous mutants and precise T-DNA insertion sites were confirmed by PCR and sequencing with primers A, C and LB1 (all primer sequences are listed in Supplementary Table S1) for ltd; primers LP54, RP54 and LB2 for ffc; and primers FtsYF, FtsYR and LB1 for cpftsY. The chaos mutant and chaos/ffc double mutant were kindly provided by Dr Laurent Nussaume 23,29 . Wild-type and homozygous mutant plants were grown on Murashige and Skoog (MS) media supplemented with 3% sucrose under short-day conditions (10 h of light/14 h of dark) with a photon flux density of 80 µmol m − 2 s − 1 in a growth chamber at 22 °C.…”

Identification of Putative Substrates of SEC2, a Chloroplast Inner Envelope Translocase

“…Mutations in components of a secondary destination translocase (e.g. the thylakoid translocases; Li et al, 1995;Voelker and Barkan, 1995;Hutin et al, 2002;Martin et al, 2009) generally have effects limited to that compartment. Although SEC2 can be considered a destination translocase, reduced SCY2 expression through Figure 9.…”

Efficient Assembly of Photosystem II in Chlamydomonas reinhardtii Requires Alb3.1p, a Homolog of Arabidopsis ALBINO3

“…The membrane insertion of the LHCPs strictly requires cpFtsY, GTP, and the integral membrane protein Alb3 in vitro (Tu et al, 1999;Moore et al, 2000Moore et al, , 2003. However, disruption of the genes encoding cpSRP54 and cpSRP43 in Arabidopsis thaliana has only mild effects on the accumulation of LHCPs in comparison with the wild type, whereas the disruption of Alb3p leads to a loss of LHCPs (Sundberg et al, 1997;Tu et al, 1999;Hutin et al, 2002). This suggests the existence of an alternative insertion pathway of LHCPs into the thylakoid membrane and/or additional functions of the mutated factors in vivo.…”