A nuclear-encoded protein of prokaryotic origin is essential for the stability of photosystem II in Arabidopsis thaliana

Abstract

To understand the regulatory mechanisms underlying the biogenesis of photosystem II (PSII) we have characterized the nuclear mutant hcf136 of Arabidopsis thaliana and isolated the affected gene. The mutant is devoid of any photosystem II activity, and none of the nuclear- and plastome-encoded subunits of this photosystem accumulate to significant levels. Protein labelling studies in the presence of cycloheximide showed that the plastome-encoded PSII subunits are synthesized but are not stable. The HCF136 gene was isolated by virtue of its T-DNA tag, and its identity was confirmed by complementation of homozygous hcf136 seedlings. Immunoblot analysis of fractionated chloroplasts showed that the HCF136 protein is a lumenal protein, found only in stromal thylakoid lamellae. The HCF136 protein is produced already in dark-grown seedlings and its levels do not increase dramatically during light-induced greening. This accumulation profile confirms the mutational data by showing that the HCF136 protein must be present when PSII complexes are made. HCF136 homologues are found in the cyanobacterium Synechocystis species PCC6803 (slr2034) and the cyanelle genome of Cyanophora paradoxa (ORF333), but are lacking in the plastomes of chlorophytes and metaphytes as well as from those of rhodo- and chromophytes. We conclude that HCF136 encodes a stability and/or assembly factor of PSII which dates back to the cyanobacterial-like endosymbiont that led to the plastids of the present photosynthetic eukaryotes.

PsbN Is Required for Assembly of the Photosystem II Reaction Center in Nicotiana tabacum

“…This is supported by the localization of the PSII assembly factor HCF136 in stroma lamellae, polysome binding to exposed thylakoids, cotranslational assembly of D1, and the preferential appearance of distinct smaller PSII subcomplexes in stroma lamellae (Yamamoto et al, 1981;Meurer et al, 1998;Zhang et al, 1999). Initial assembly of thylakoid membrane complexes occurs through a number of intermediate steps, which follow a strict hierarchical and concerted order, and is to some extent also translationally regulated (Zhang et al, 2000;Choquet and Wollman, 2002).…”

“…For instance, HCF136 was shown to be required primarily for accumulation of PSII (Meurer et al, 1998). Although translation rates of PSII proteins in hcf136 were unaffected, their stationary levels were barely detectable, causing seedling lethality.…”

“…Although translation rates of PSII proteins in hcf136 were unaffected, their stationary levels were barely detectable, causing seedling lethality. HCF136 is found in the nonappressed stroma lamellae and is essential for early assembly of the PSII RC (Meurer et al, 1998;Plücken et al, 2002). LPA19, a Psb27 homolog, is suggested to facilitate processing of the D1 precursor protein at the C terminus at the lumenal side by the CtpA protease after insertion into the complex to generate mature D1 (Wei et al, 2010;Chi et al, 2012a).…”

BIOGENESIS FACTOR REQUIRED FOR ATP SYNTHASE 3 Facilitates Assembly of the Chloroplast ATP Synthase Complex

“…Chloroplast protein labeling and chasing were performed as previously described (Meurer et al, 1998), with the following modifications. Primary leaves of 12-d-old young seedlings were preincubated in buffer containing 20 mg/ml cycloheximide for 30 min and radiolabeled with 1 mCi/mL [ 35 S]-Met in the presence of cycloheximide for 20 min at 80 mmol photons m 22 s 21 light intensity.…”

Regulatory factors for the assembly of thylakoid membrane protein complexes

“…HCF136 (high chlorophyll fluorescence 136), which encodes a hydrophilic protein localized in the lumen of stroma thylakoids, was first identified as being involved in PSII assembly in Arabidopsis through isolation and characterization of high chlorophyll fluorescence mutants [13]. In the absence of HCF136, PSII proteins are synthesized normally but do not assemble into a stable PSII complex [13]. Under standard illumination, the amounts of both PSII and PSI are reduced, suggesting that HCF136 may be required for photosystem biogenesis in general [13].…”