Envelope Membrane Proteins That Interact with Chloroplastic Precursor Proteins

Abstract

The post-translational transport of cytoplasmically synthesized precursor proteins into chloroplasts requires proteins in the envelope membranes. To identify some of these proteins, label transfer cross-linking was performed using precursor to the small subunit of ribulose-1,5-bisphosphate carboxylase (prSSU) that was blocked at an early stage of the transport process. Two envelope proteins were identified: an 86-kD protein and a 75-kD protein, both present in the outer membrane. Labeling of both proteins required prSSU and could not be accomplished with SSU lacking a transit peptide. Labeling of the 75-kD protein occurred only when low levels of ATP were present, whereas labeling of the 86-kD protein occurred in the absence of exogenous ATP. Although both labeled proteins were identified as proteins of the outer envelope membrane, the labeled form of the 75-kD protein could only be detected in fractions containing mixed envelope membranes. Based on these observations, we propose that prSSU first binds in an ATP-independent fashion to the 86-kD protein. The energy-requiring step is association with the 75-kD protein and assembly of a translocation contact site between the inner and outer membrane of the chloroplastic envelope.

The Arabidopsisppi1Mutant Is Specifically Defective in the Expression, Chloroplast Import, and Accumulation of Photosynthetic Proteins[W]

“…In support of this model, preproteins can be cross-linked to pea Toc34 (psToc34) and/or psToc159 very early during import into pea chloroplasts (Perry and Keegstra, 1994;Ma et al, 1996;Kouranov and Schnell, 1997), and a direct and specific interaction between preproteins and psToc34 has been observed in vitro (Sveshnikova et al, 2000;Schleiff et al, 2002). However, not all cross-linking studies resulted in the identification of psToc34 (Perry and Keegstra, 1994;Ma et al, 1996), and in those that did, psToc34 appeared to interact with the mature region of the preprotein rather than with the transit peptide, as would be expected of a receptor (Kouranov and Schnell, 1997).…”

“…The core components of the Toc complex are called Toc159, Toc34, and Toc75, according to their molecular weights (Hirsch et al, 1994;Kessler et al, 1994;Perry and Keegstra, 1994;Schnell et al, 1994;Seedorf et al, 1995;Bölter et al, 1998). Toc159 and Toc34 are related GTPases that have been proposed to act as preprotein receptors.…”

AtToc90, a New GTP-Binding Component of the Arabidopsis Chloroplast Protein Import Machinery

“…The available evidence indicates that the two proteins act in concert to recognize the chloroplast targeting peptide (Kouranov and Schnell, 1997;Sveshnikova et al, 2000;Chen et al, 2000a). Toc159 is thought to act as the primary receptor of transit sequences (Hirsch et al, 1994;Perry and Keegstra, 1994;Kouranov and Schnell, 1997). An alternative model in which Toc34 functions as the primary receptor and Toc159 as a GTP-dependent translocation motor has also been proposed (Schleiff et al, 2003).…”

The Omp85-Related Chloroplast Outer Envelope Protein OEP80 Is Essential for Viability in Arabidopsis

“…It associates with precursor proteins in vitro (Perry and Keegstra, 1994;Schnell et al, 1994), and was reconstituted as a cationselective ion channel in artificial liposomes (Hinnah et al, 1997), suggesting that it forms a major component of the preprotein translocation channel (Bédard and Jarvis, 2005;Kessler and Schnell, 2006;Smith, 2006). Unlike other outer membrane proteins, Toc75 is synthesized as a larger precursor with a bipartite targeting signal (Tranel et al, 1995;Tranel and Keegstra, 1996); the first part is a standard transit peptide for chloroplast import (Inoue et al, 2001) and the second part acts as an intraorganellar targeting signal that is cleaved by an envelope-bound type I signal peptidase (Inoue and Keegstra, 2003;Inoue et al, 2005;Baldwin and Inoue, 2006).…”