Import of the barley PSI-F subunit into the thylakoid lumen of isolated chloroplasts

Abstract

A full-length cDNA clone encoding the PSI-F subunit of barley photosystem I has been isolated and sequenced. The open reading frame encodes a precursor polypeptide with a deduced molecular mass of 24837 Da. The barley PSI-F precursor contains a bipartite presequence with characteristics similar to the presequences of proteins destined to the thylakoid lumen. In vitro import studies demonstrate that an in vitro synthesized precursor is transported across the chloroplast envelope and directed to the thylakoid membrane, where it accumulates in a protease-resistant form. Incubation of the precursor with a chloroplast stromal extract results in processing to a form intermediate in size between the precursor and mature forms. Hydrophobicity analysis of the barley PSI-F protein reveals a hydrophobic region predicted to be a membrane spanning alpha-helix. The hydrophobic nature of PSI-F combined with a bipartite presequence is unusual. We postulate that the second domain in the bipartite presequence of the PSI-F precursor proteins is required to ensure the proper orientation of PSI-F in the thylakoid membrane. The expression of the PsaF gene is light-induced similar to other barley photosystem I genes.