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

Abstract

Thylakoid membrane-localized chloroplast ATP synthases use the proton motive force generated by photosynthetic electron transport to produce ATP from ADP. Although it is well known that the chloroplast ATP synthase is composed of more than 20 proteins with a 3 b 3 g 1 « 1 d 1 I 1 II 1 III 14 IV 1 stoichiometry, its biogenesis process is currently unclear. To unravel the molecular mechanisms underlying the biogenesis of chloroplast ATP synthase, we performed extensive screening for isolating ATP synthase mutants in Arabidopsis (Arabidopsis thaliana). In the recently identified bfa3 (biogenesis factors required for ATP synthase 3) mutant, the levels of chloroplast ATP synthase subunits were reduced to approximately 25% of wild-type levels. In vivo labeling analysis showed that assembly of the CF 1 component of chloroplast ATP synthase was less efficient in bfa3 than in the wild type, indicating that BFA3 is required for CF 1 assembly. BFA3 encodes a chloroplast stromal protein that is conserved in higher plants, green algae, and a few species of other eukaryotic algae, and specifically interacts with the CF 1 b subunit. The BFA3 binding site was mapped to a region in the catalytic site of CF 1 b. Several residues highly conserved in eukaryotic CF 1 b are crucial for the BFA3-CF 1 b interaction, suggesting a coevolutionary relationship between BFA3 and CF 1 b. BFA3 appears to function as a molecular chaperone that transiently associates with unassembled CF 1 b at its catalytic site and facilitates subsequent association with CF 1 a during assembly of the CF 1 subcomplex of chloroplast ATP synthase.

Nucleus-Encoded Protein BFA1 Promotes Efficient Assembly of the Chloroplast ATP Synthase Coupling Factor 1

“…The main component of NPQ, qE, is strictly dependent on the level of protons in the thylakoid lumen, which in turn is mainly controlled by photosynthetic electron transport activity and chloroplast ATP synthase. In the mutants with reduced ATP synthase activity, proton efflux from thylakoid lumen is less efficient, resulting in a high level of NPQ (Zhang et al, 2016). The bfa1-1 mutant was isolated on the basis of its high-NPQ phenotype upon illumination, as previously described ( Figure 1A; Zhang et al, 2016).…”

“…In the mutants with reduced ATP synthase activity, proton efflux from thylakoid lumen is less efficient, resulting in a high level of NPQ (Zhang et al, 2016). The bfa1-1 mutant was isolated on the basis of its high-NPQ phenotype upon illumination, as previously described ( Figure 1A; Zhang et al, 2016). The bfa1-2 line was obtained from the NASC (European Arabidopsis Stock Centre).…”

“…In chloroplasts, in vitro reconstitution studies suggest that the formation of the α/β dimer, which involves several chaperones, including Cpn60, Cpn24, and Hsp70, is the first step of CF 1 assembly (Chen and Jagendorf, 1994). An analysis of Arabidopsis thaliana mutants revealed that the chloroplast stromal proteins PAB (PROTEIN IN CHLOROPLAST ATPASE BIOGENESIS) and BFA3 (also known as CONSERVED IN THE GREEN LINEAGE AND DIATOMS11) are involved in the assembly of the CF 1 subcomplex by specifically interacting with CF 1 γ and CF 1 β, respectively (Mao et al, 2015;Grahl et al, 2016;Zhang et al, 2016). However, because the whole assembly process of the CF 1 module is still unclear, we isolated mutants that accumulated reduced amounts of chloroplast ATP synthase, with the goal of identifying proteins required for its assembly.…”

Recent advances in understanding photosynthesis

“…While for PSI and NDH, and particularly PSII, substantial sets of assembly factors have been known for many years already, until recently only one auxiliary protein (ALB4 19 ) involved in the biogenesis of the chloroplast ATP synthase (cpATPase) has been isolated. Three additional cpATPase assembly factors have now been identified: CGL160 20, 21 , PAB 22 , and CGLD11/BFA3 23, 24 . CGL160 represents a protein of prokaryotic origin, whereas CGLD11/BFA3 and PAB are specific to photosynthetic eukaryotes.…”

Assembling a Nanomolecular Power Station

“…Five different types of subunits (α, β, γ, δ, and ε) assemble precisely to form the CF 1 component of the mature CF 1 CF o ATP synthase, but the assembly mechanism is poorly understood. An extensive screen had identified Arabidopsis thaliana mutants showing the high level of nonphotochemical quenching that is typical of ATP synthase mutants (Zhang et al, 2016). These biogenesis factor required for ATP synthase (bfa) mutants are deficient in the assembly of the chloroplast ATP synthase.…”