Shedding light on the chloroplast protein import machinery

Abstract

Department of Biological Sciences Rutgers The State University of New Jersey Newark, New Jersey 07102 The complex architecture of the chloroplast presents an intriguing challenge for investigators interested in the mechanisms of protein targeting and organelle biogene- sis. Chloroplasts are subdivided by three noncontiguous membrane systems into at least six suborganellar com- partments that serve to segregate and organize a number of essential metabolic functions, most notably the reac- tions of photosynthesis. The development and mainte- nance of this structure require an elaborate sorting system to ensure proper targeting and integration of resident pro- teins. This complexity is compounded by the fact that the vast majority of chloroplast polypeptides are nu- clear-encoded and posttranslationally imported into the organelle from the cytosol. This cascade of targeting and assembly reactions is initiated by the recognition and translocation of precursor proteins at the double mem- brane of the chloroplast envelope. After years of studies that have contributed to our under- standing of the general pathway of import, recent investi- gations have begun to uncover the components that un- derlie the mechanism of targeting and translocation at the envelope. These studies support the existence of a single common mechanism of envelope transport for all cytosoli- tally synthesized precursors that are destined forthe inter- nal compartments of the chloroplast. In particular, exten- sive progress has been made in understanding the import apparatus in the outer membrane of the envelope. There is now convincing evidence that the initial stages of envelope translocation are mediated by a complex of outer envelope membrane proteins (OEPs) that includes two GTP-binding proteins, a putative channel component, and at least two membrane-associated molecular chaperones. Several candidates for components of the inner membrane trans- location machinery also have been identified, and their structures and specific functions are under investigation. These recent discoveries are especially satisfying be- cause they corroborate previous evidence supporting a single mechanism of envelope translocation. Moreover, the apparent activities of the components serve as the basis for the formulation of testable hypotheses that should greatly facilitate dissection of the mechanism of envelope translocation. This minireview will focuson these recent discoveries and their implications with respect to the mechanism of translocation in chloroplasts and other relevant systems. More detailed discussions of the import process can be found in recent review articles (Gray and Row, 1995; Keegstra et al., 1995). Identification of Envelope Translocation Components Chloroplast protein import can be divided into two distinct steps (reviewed by Keegstra et al., 1995). In the first step,