Site-directed mutagenesis and analysis of second-site revertants indicates a requirement for C-terminal amino acids of PsaB for stable assembly of the photosystem I reaction center complex in Chlamydomonas reinhardtii.
The PsaA and PsaB polypeptides form the reaction center core heterodimer of photosystem I (PSI). Both PsaA and PsaB are predicted to have 11 hydrophobic domains, although it is unclear how both polypeptides fold within the thylakoid membrane. If all 11 hydrophobic regions form membrane-spanning domains, the N- and C-terminus must be located on opposite sides of the membrane. The C-terminus of PsaB is very conserved in a wide range of organisms and may be important for PSI assembly or function. Using chloroplast transformation in Chlamydomonas reinhardtii we have generated a series of C-terminal extension and deletion mutants of the PsaB polypeptide. Analysis of these mutants and spontaneous revertants indicates that the C-terminus may be extended by at least 14 amino acids without impairing PSI assembly. Deletion of amino acids 732-736 also has no impact on PSI, whereas deletion of amino acids 727-736 results in no accumulation of the complex. The site of truncation in the 727-736 deletion coincides with the end of the hydrophobic domain XI supporting a location of the C-terminus of PsaB on the lumenal side of PSI.
Lee
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Department of Botany, Arizona State University, Tempe 85287-1601, USA.
Bingham
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Webber
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D000595:Amino Acid Sequence; D000818:Animals; D001483:Base Sequence; D016825:Chlamydomonas reinhardtii / Q000737:chemistry / Q000235:genetics*; D017124:Conserved Sequence; D016054:DNA, Protozoan / Q000235:genetics; D008565:Membrane Proteins / Q000737:chemistry / Q000235:genetics*; D008958:Models, Molecular; D008969:Molecular Sequence Data; D016297:Mutagenesis, Site-Directed; D010777:Photochemistry; D045322:Photosynthetic Reaction Center Complex Proteins / Q000737:chemistry / Q000235:genetics*; D045331:Photosystem I Protein Complex; D017510:Protein Folding; D017386:Sequence Homology, Amino Acid
D016054:DNA, Protozoan; D008565:Membrane Proteins; D045322:Photosynthetic Reaction Center Complex Proteins; D045331:Photosystem I Protein Complex; C089909:photosystem I, psaB subunit