Identification of protein W, the elusive sixth subunit of the Rhodopseudomonas palustris reaction center-light harvesting 1 core complex

Philip J. Jackson, Andrew Hitchcock, David J.K. Swainsbury, Pu Qian, Elizabeth C. Martin, David A. Farmer, Mark J. Dickman, Daniel P. Canniffe, C. Neil Hunter

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The X-ray crystal structure of the Rhodopseudomonas (Rps.) palustris reaction center-light harvesting 1 (RC-LH1) core complex revealed the presence of a sixth protein component, variably referred to in the literature as helix W, subunit W or protein W. The position of this protein prevents closure of the LH1 ring, possibly to allow diffusion of ubiquinone/ubiquinol between the RC and the cytochrome bc1 complex in analogous fashion to the well-studied PufX protein from Rhodobacter sphaeroides. The identity and function of helix W have remained unknown for over 13 years; here we use a combination of biochemistry, mass spectrometry, molecular genetics and electron microscopy to identify this protein as RPA4402 in Rps. palustris CGA009. Protein W shares key conserved sequence features with PufX homologs, and although a deletion mutant was able to grow under photosynthetic conditions with no discernible phenotype, we show that a tagged version of protein W pulls down the RC-LH1 complex. Protein W is not encoded in the photosynthesis gene cluster and our data indicate that only approximately 10% of wild-type Rps. palustris core complexes contain this non-essential subunit; functional and evolutionary consequences of this observation are discussed. The ability to purify uniform RC-LH1 and RC-LH1-protein W preparations will also be beneficial for future structural studies of these bacterial core complexes.

Original languageEnglish
Pages (from-to)119-128
Number of pages10
JournalBiochimica et Biophysica Acta - Bioenergetics
Issue number2
Publication statusPublished - Feb 2018


  • Helix W
  • Photosynthesis
  • PufX
  • Reaction center-light harvesting 1 (RC-LH1) core complex
  • Rhodopseudomonas palustris
  • RPA4402

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