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.

Centre Algatech Laboratory of Photosynthesis Institute of Microbiology of the Czech Academy of Sciences Opatovický mlýn 37981 Třeboň Czech Republic

Laboratory of Molecular Structure Characterization Institute of Microbiology of the Czech Academy of Sciences 14220 Praha 4 Krč Czech Republic

Sir Ernst Chain Building Wolfson Laboratories Department of Life Sciences South Kensington Campus Imperial College London London SW7 2AZ UK

The Pasarow Mass Spectrometry Laboratory The Jane and Terry Semel Institute for Neuroscience and Human Behavior David Geffen School of Medicine UCLA Los Angeles CA 90095 USA

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