PucC and LhaA direct efficient assembly of the light‐harvesting complexes in Rhodobacter sphaeroides

Abstract

SummaryThe mature architecture of the photosynthetic membrane of the purple phototroph R hodobacter sphaeroides has been characterised to a level where an atomic‐level membrane model is available, but the roles of the putative assembly proteins LhaA and PucC in establishing this architecture are unknown. Here we investigate the assembly of light‐harvesting LH2 and reaction centre‐light‐harvesting1‐PufX (RC‐LH1‐PufX) photosystem complexes using spectroscopy, pull‐downs, native gel electrophoresis, quantitative mass spectrometry and fluorescence lifetime microscopy to characterise a series of lha A and puc C mutants. LhaA and PucC are important for specific assembly of LH1 or LH2 complexes, respectively, but they are not essential; the few LH1 subunits found in Δlha A mutants assemble to form normal RC‐LH1‐PufX core complexes showing that, once initiated, LH1 assembly round the RC is cooperative and proceeds to completion. LhaA and PucC form oligomers at sites of initiation of membrane invagination; LhaA associates with RCs, bacteriochlorophyll synthase (BchG), the protein translocase subunit YajC and the YidC membrane protein insertase. These associations within membrane nanodomains likely maximise interactions between pigments newly arriving from BchG and nascent proteins within the SecYEG‐SecDF‐YajC‐YidC assembly machinery, thereby co‐ordinating pigment delivery, the co‐translational insertion of LH polypeptides and their folding and assembly to form photosynthetic complexes.

Genes essential for phototrophic growth by a purple alphaproteobacterium

“…This gene, which has recently been shown to be involved in LH1 assembly in Rba. sphaeroides (Mothersole et al ., ), did not appear as an essential gene in our Tn‐seq analysis, but as shown in Fig. B, a Tn mutant of lhaA grew very poorly under phototrophic conditions.…”

The PufX quinone channel enables the light‐harvesting 1 antenna to bind more carotenoids for light collection and photoprotection

“…The PufX-minus RC-LH1 complexes in the DM1R mutant assemble closed LH1 a 16 b 16 rings because co-operative forces between the RC and LH1 a 1 b 1 BChl 2 subunits drive the process of encircling RCs with LH1 a 1 b 1 BChl 2 Crt 2 units to completion, at the expense of some 'free' RCs that have no LH1 subunits. This LH1 ring completion process is observed even under quite extreme circumstances; for example, only~14% of the LH1a polypeptide survives deletion of the LH1 assembly factor LhaA yet complexes still assemble with a normal RC : LH1 stoichiometry, leaving a substantial population of 'free' RCs [21]. In summary, we propose that the LH1 ring of the PufX À LH1aW -24 F mutant DM1R completely surrounds the RC, on the basis of the near-identical RC : LH1 absorption ratios for DM1R and the control strain DO10 (Fig.…”

Biosynthesis of the modified tetrapyrroles—the pigments of life

“…In Rba. sphaeroides BChl synthase associates with the protein translocase subunit YajC and the YidC membrane protein insertase, and the assembly factor for the light-harvesting 1 complex, LhaA (302). It was proposed that membrane nanodomains foster interactions between pigments produced by BchG and nascent proteins within the SecYEG-SecDF-YajC-YidC assembly machinery, in order to co-ordinate assembly of this light harvesting complex.…”