PROTEIN IMPORT INTO CHLOROPLASTS

Most chloroplastic proteins are encoded in the nucleus, synthesized on cytosolic ribosomes and subsequently imported into the organelle, In general, proteins destined for the chloroplast are synthesized as prec ursor proteins with a cleavable N-terminal presequence that mediates r outing to the inside of the chloroplast. These precursor proteins have to be targeted to the correct organellar membrane surface after their release from the ribosome and furthermore they have to be maintained in a conformation suitable for translocation across the two envelope m embranes. Recognition and import of most chloroplastic precursor prote ins are accomplished by a jointly used translocation apparatus. Differ ent but complementary studies of several groups converged recently in the identification of the outer envelope proteins OEP86, OEP75, OEP70 (a Hsp70-related protein), OEP34, and of the inner envelope protein IE P110 as components of this translocation machinery. None of these prot eins, except for OEP70, shows any homology to components of other prot ein translocases. The plastid import machinery thus seems to be an ori ginal development in evolution. Following translocation into the organ elle, chloroplastic proteins are sorted to their suborganellar destina tion, i.e., the inner envelope membrane, the thylakoid membrane, and t he thylakoid lumen. This structural and evolutionary complexity of chl oroplasts is reflected by a variety of routing mechanisms by which pro teins reach their final location once inside the organelle. This revie w will focus on recent advances in the identification of components of the chloroplastic protein import machinery, and new insights into the pathways of inter and intraorganellar sorting.