A Receptor Component of the Chloroplast Protein Translocation Machinery

Abstract

The chloroplast outer envelope protein OEP86 functions as a receptor in precursor protein translocation into chloroplasts. Sequence analysis suggests that the precursor of OEP86 is directed to the chloroplast outer envelope by a cleavable, negatively charged, and unusually long amino-terminal peptide. This presequence is unlike other potential targeting signals and suggests the existence of another membrane insertion pathway. Insertion of precursor OEP86 required the hydrolysis of adenosine triphosphate and the existence of surface exposed chloroplast membrane components, and it was not competed by another precursor protein destined for the internal plastid compartments.

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

“…In pea, the Toc-complex consists of three major components forming a stable complex Ma et al, 1996). Toc159 (the number indicates the molecular mass in kD) and Toc34 are surface exposed, GTP-binding integral membrane proteins (Hirsch et al, 1994;Kessler et al, 1994;Seedorf et al, 1995;Chen et al, 2000a). The two proteins share a highly conserved GTP-binding domain .…”

“…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 A-and G-domains are both exposed to the cytosol (Chen et al, 2000a). A C-terminal Mdomain (membrane anchor domain) lacking predicted hydrophobic transmembrane helices anchors the proteins in the outer membrane (Hirsch et al, 1994;Kessler et al, 1994). In addition, precursor proteins were shown to directly interact with the M-domain (Kouranov and Schnell, 1997).…”

The gateway to chloroplast: re-defining the function of chloroplast receptor proteins

“…Toc159, on the other hand, can be subdivided into three functional domains: an acidic amino acid extension at the N-terminus (A-domain), a central GTP-binding domain (G-domain) and a C-terminal membrane anchoring domain of 52 kDa (M-domain) (Muckel and Soll , 1996 ). Both the A-and G-domain are exposed to the cytosol while the M-domain substantiates the membrane anchoring (Hirsch et al , 1994 ). While the G-and M-domain between the different homologs exhibit a relatively high homology, their A-domain has fairly low sequence conservation (Ivanova et al , 2004 ).…”

“…Among the first components of the chloroplast import machinery to be identified were three components of the outer membrane of pea chloroplasts (Schnell and Blobel , 1993 ;Waegemann and Soll , 1995 ). These include a β -barrel membrane channel, Toc75 Keegstra and Cline , 1999 ), and associated its associated receptor constituents Toc159 and Toc34 (Hirsch et al , 1994 ;Kessler, Blobel et al, 1994;Seedorf et al , 1995 ). Both receptors are integral GTPases at the outer membrane.…”