Identification of a signal that distinguishes between the chloroplast outer envelope membrane and the endomembrane system in vivo

Certain small outer envelope membrane proteins of chloroplasts are encoded by the nuclear genome without a cleavable N-terminal transit peptide. We investigated in vivo the targeting mechanism of AtOEP7, an Arabidopsis homolog of the small outer envelope membrane protein. AtOEP7 was expressed as a fusion protein with the green fluorescent protein (GFP) either transiently in protoplasts or stably in transgenic plants. In either case, fluorescence microscopy of transformed cells and protein gel blot analysis of fractionated proteins confirmed that the AtOEP7:GFP fusion protein was targeted to the chloroplast outer envelope membrane. In vivo targeting experiments revealed that two regions, the transmembrane domain (TMD) and its C-terminal neighboring seven-amino acid region, were necessary and sufficient for targeting to the chloroplast outer membrane. Substitution of aspartic acid or lysine residues with glycine residues or scrambling of the amino acid sequence of the seven-amino acid region caused mistargeting to the plasma membrane. Although the amino acid sequence of the TMD is not important for targeting, amino acid residues with large side chains inhibited targeting to the chloroplasts and resulted in the formation of large aggregates in the protoplasts. In addition, introduction of a proline residue within the TMD resulted in inhibition of targeting. Finally, a fusion protein, AtOEP7:NLS:GFP, was targeted efficiently to the chloroplast envelope membranes despite the presence of a nuclear localization signal. On the basis of these results, we conclude that the seven-amino acid region and the TMD are determinants for targeting to the chloroplast outer envelope membrane. The seven-amino acid region plays a critical role in AtOEP7 evading the endomembrane system and entering the chloroplast pathway, and the TMD plays critical roles in migration to the chloroplasts and/or subsequent insertion into the membrane.