ABSTRACT
Molecular chaperones seem to accompany plastid destined precursor proteins from their origin in the cytosol until they reach the outmost corner of the organelle, e.g. the thylakoid lumen. By then they will have encountered cytosolic, envelope membrane bound, stromal and lumenal chaperones of either the HSP70, Cpn60/ Cpn10, Sec or SRP family. While none of these systems is unique to plastids, the wide variety which is present simultaneously is surprising. This review will therefore focus on the diversity of chaperones involved in protein translocation and trafficking in plastids and point to the
1. Introduction 2. Cytosolic Factors Involved in the Translocation of Precursor Proteins into Chloroplasts 2.1. Conformation of Precursor Proteins and Cytosolic Factors 2.2. Conformational Changes of Precursor Proteins at the Chloroplast Envelope 3. Molecular Chaperones in the Translocation Machinery of the Outer Envelope of Chloroplasts 3.1. Components of the Translocation Machinery 3.2. Characteristics of HSP70 Homologues Present in the Outer Envelope of Chloroplasts 4. Stromal Chaperones 4.1. Stromal HSP70 Homologues 4.2. Chloroplast Chaperonins 5. Role of Stromal Factors in the Translocation of Proteins into or Across the Thylakoid Membrane 5.1. Targeting to the Thylakoid Membrane 5.2. Targeting to the Thylakoid Lumen 6. Molecular Chaperones in the Thylakoid Lumen 7. References
uniqueness of several of the involved systems in comparison to other organelles and organisms. Excellent reviews on the mechanistic background of chaperone functions have appeared very recently and the reader is referred to them for further details (Gatenby and Viitanen, 1994; Hartl, 1996; Boston et al., 1996; Gatenby, 1996).
