Protein import into chloroplasts

Abstract

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 precursor proteins with a cleavable N‐terminal presequence that mediates routing 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 membranes. Recognition and import of most chloroplastic precursor proteins are accomplished by a jointly used translocation apparatus. Different but complementary studies of several groups converged recently in the identification of the outer envelope proteins OEP86, OEP75, OEP70 (a Hsp 70‐related protein), OEP34, and of the inner envelope protein IEP110 as components of this translocation machinery. None of these proteins, except for OEP70, shows any homology to components of other protein translocases. The plastid import machinery thus seems to be an original development in evolution. Following translocation into the organelle, chloroplastic proteins are sorted to their suborganellar destination, i.e., the inner envelope membrane, the thylakoid membrane, and the thylakoid lumen. This structural and evolutionary complexity of chloroplasts is reflected by a variety of routing mechanisms by which proteins reach their final location once inside the organelle. This review 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.

A High-Conductance Solute Channel in the Chloroplastic Outer Envelope from Pea

“…Solute transport, including protein transport, is a major task of the outer envelope membrane. Thus, proteins involved in transport should be prominent in this membrane, as has been shown for subunits of the protein translocon of chloroplastic outer envelopes Toc86, Toc75, and Toc34 (reviewed in Lübeck et al, 1997;Heins et al, 1998) or the amino acidselective channel protein OEP16. An additional criterion that is common to many channel proteins is their resistance to proteolysis.…”

The chloroplastic protein import machinery contains a Rieske-type iron-sulfur cluster and a mononuclear iron-binding protein

“…Accordingly, the targeting and translocation of chloroplast‐destined precursor proteins plays a central role during biogenesis of the highly structured chloroplasts. The recognition and transport processes of the precursor proteins involve the cooperation of joint translocation sites localized in both the outer and inner envelope membranes (for reviews, see Fuks and Schnell 1997; Lübeck et al ., 1997). Very recently, a uniform nomenclature for proteins of the chloroplast protein import machinery has been introduced (Schnell et al ., 1997).…”

A Chromodomain Protein Encoded by the Arabidopsis CAO Gene Is a Plant-Specific Component of the Chloroplast Signal Recognition Particle Pathway That Is Involved in LHCP Targeting

“…Thylakoid proteins are encoded by both the nuclear and chloroplast genomes. Nuclear-encoded proteins destined for the thylakoid membrane are synthesized with a cleavable N-terminal extension (transit peptide) that targets the protein across the envelope membranes into the stroma (reviewed in Cline and Henry, 1996;Kermode, 1996;Lübeck et al, 1997). Multiple mechanisms exist for targeting proteins from the stroma to the thylakoid membrane (reviewed in Cline and Henry, 1996;Klösgen, 1997).…”