Protein Targeting to the Chloroplast Outer Membrane

Nuclear-encoded chloroplast proteins can be roughly divided into two groups. Proteins in the first group are synthesized as higher molecular weight precursors with N-terminal extensions called transit peptides. This group of proteins includes all proteins destined for the interior of chloroplasts and two proteins destined for the outer envelope membrane. The second group of proteins consists of the rest of outer membrane proteins identified so far. These outer membrane proteins are synthesized at their mature size in the cytosol without a cleavable transit peptide. From the second group of proteins, we have chose a protein, OEP14 (1), to study the mechanism of protein import to the outer membrane. The outer membrane targeting/insertion signal of OEP14 has been localized to the N-terminal first 30 amino acids of the protein. This signal shares some features with the ER-targeting signal peptides but is nevertheless specific for the chloroplastic outer membrane (2). Insertion of OEP14 to the outer membrane require a trypsin-sensitive and an NEM-sensitive components.