Abstract
Photochemical energy conversion during oxygenic photosynthesis is performed by membrane-embedded chlorophyll-binding protein complexes. The biogenesis and maintenance of these complexes requires auxiliary protein factors that optimize the assembly process and protect nascent complexes from photodamage. In cyanobacteria, several lipoproteins contribute to the biogenesis and function of the photosystem II (PSII) complex. They include CyanoP, CyanoQ, and Psb27, which are all attached to the lumenal side of PSII complexes. Here, we show that the lumenal Ycf48 assembly factor found in the cyanobacterium Synechocystis sp. PCC 6803 is also a lipoprotein. Detailed mass spectrometric analysis of the isolated protein supported by site-directed mutagenesis experiments indicates lipidation of the N-terminal C29 residue of Ycf48 and removal of three amino acids from the C-terminus. The lipobox sequence in Ycf48 contains a cysteine residue at the −3 position compared to Leu/Val/Ile residues found in the canonical lipobox sequence. The atypical Ycf48 lipobox sequence is present in most cyanobacteria but is absent in eukaryotes. A possible role for lipoproteins in the coordinated assembly of cyanobacterial PSII is discussed.
Highlights
During oxygenic photosynthesis, photosystem II (PSII) and photosystem I (PSI) complexes utilize light to extract electrons from water to produce the NADPH and ATP needed for CO2 fixation and the synthesis of sugars
Three lumenal accessory factors involved in the assembly and maintenance of cyanobacterial PSII have been identified as lipoproteins [10]
They include Psb27, CyanoP, and CyanoQ, which is found in oxygen-evolving PSII complexes but could act as an assembly factor during the later stages of PSII biogenesis [21]
Summary
Photosystem II (PSII) and photosystem I (PSI) complexes utilize light to extract electrons from water to produce the NADPH and ATP needed for CO2 fixation and the synthesis of sugars The biogenesis of both complexes requires an array of auxiliary protein factors that generally optimize the process, enable the correct binding of cofactors, and protect the assembly intermediates from photodamage [1,2,3,4]. The signal peptide is cleaved within a conserved motif of [LVI]−3 [ASTVI]−2 [GAS]−1 [C]1, where [C]1 designates the starting cysteine (C) residue of the apoprotein (Figure 1a). This four-residue motif is called the lipobox and is conserved in eubacteria. Prior to cleavage of the signal peptide, the SH group of [C]1 is modified by the attachment of a diacylglycerol moiety, which is transferred from phosphatidylglycerol (PG)
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