Subunit G of photosystem I is a nuclear-encoded protein, predicted to form two transmembrane a-helices separated by a loop region. We use in vitro import assays to show that the positively charged loop domain faces the stroma, whilst the N- and C-termini most likely face the lumen. PSI-G constructs in which a His- or Strep-tag is placed at the C-terminus or in the loop region insert with the same topology as wild-type photosys-tem I subunit G (PSI-G).