FtsH homolog in chloroplasts

This chapter describes the structural chemistry and the biological aspects of FtsH homolog in chloroplasts. Chloroplast FtsH is named after its well-characterized Escherichia coli homolog. It was first identified and characterized in spinach chloroplasts using antibodies against a highly conserved synthetic peptide, and cloned from an Arabidopsis thaliana cDNA library. Chloroplast FtsH, like its bacterial and mitochondrial relatives, is a membrane-bound, ATP-dependent metalloprotease. It is located in the thylakoid membrane, with its ATPase and catalytic domains exposed to the soluble stroma. Recombinant chloroplast FtsH, expressed as a GST-fusion protein, remains water-soluble and can degrade the model substrate β-casein in an ATP-stimulated manner. It is sensitive to the metalloprotease inhibitor 1,10-phenanthroline, but insensitive to a cocktail of serine and cysteine protease inhibitors. The deduced amino acid sequences of chloroplast FtsHs suggest that they are synthesized as precursor proteins with N-terminal extensions, which serve as a chloroplast targeting signal and are removed after translocation of the protein into the organelle.