A Toc75‐like protein import channel is abundant in chloroplasts

Abstract

Chloroplasts import post-translationally most of their constituent polypeptides via two distinct translocon units located in the outer and inner envelope. The protein import channel of the translocon of the outer envelope of chloroplasts, Toc75, is the most abundant protein in that membrane. We identify a novel Toc75 homologous protein, atToc75-V, a prominent protein that is clearly localized in the chloroplastic outer envelope. Phylogenetic analysis indicates that Toc75-V is more closely related to its prokaryotic ancestors than to Toc75 from plants. The presence of a second translocation channel suggests that alternative, previously unrecognized import routes into chloroplasts might exist.

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

“…A putative pea ortholog of atToc75-V was not associated with the protein translocation machinery, suggesting that its role is not directly related to chloroplast protein import (Eckart et al, 2002). The pea protein appeared to be smaller (apparent size approximately 66 kD) than the conceptual translation of atTOC75-V (approximately 80 kD), whereas the latter was predicted to carry a transit peptide.…”

“…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). Arabidopsis (Arabidopsis thaliana) possesses four genomic sequences with homology to psToc75 on chromosomes I, III, IV, and V; these are termed atTOC75-I, -III, -IV, and -V, respectively (Jackson- Constan and Keegstra, 2001;Eckart et al, 2002). Among them, atTOC75-I, -III, and -IV are highly homologous to one another and to the pea gene, with predicted amino acid sequence identities ranging from 60% to 75%.…”

“…The fourth Omp85 homolog of Arabidopsis chloroplasts, atToc75-V, is more distantly related to psToc75, sharing only 22% identity (Eckart et al, 2002). A putative pea ortholog of atToc75-V was not associated with the protein translocation machinery, suggesting that its role is not directly related to chloroplast protein import (Eckart et al, 2002).…”

“…The pea protein appeared to be smaller (apparent size approximately 66 kD) than the conceptual translation of atTOC75-V (approximately 80 kD), whereas the latter was predicted to carry a transit peptide. Thus, it was proposed that atToc75-V has an 11-kD targeting sequence that is removed to yield a mature protein of 69 kD (Eckart et al, 2002). However, subsequent work suggested that atToc75-V (a polypeptide of 732 residues, as encoded by the originally annotated open reading frame) was targeted to isolated chloroplasts without undergoing any change in size (Inoue and Potter, 2004).…”

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

“…The Arabidopsis genome encodes two paralogs of Toc34 (atToc33 and atToc34); Gutensohn et al , 2000 ), and four paralogs of Toc159 (atToc159, atToc132, atToc120 and atToc90; Bauer et al , 2000 ;Hiltbrunner et al , 2004 ;Kubis et al , 2004 ) and Toc75 (atToc75-III, atToc75-IV, atToc75-I and atToc75V/ atOep80) (Eckart et al , 2002 ;Baldwin et al, 2005). The discovery of multiple arrays of isoforms in Arabidopsis promises to make the functional characterization of each individual component of the chloroplast import system challenging.…”