Dual Targeting of a Processing Peptidase into Both Endosymbiotic Organelles Mediated by a Transport Signal of Unusual Architecture

Abstract

As a result of the endosymbiotic gene transfer, the majority of proteins of mitochondria and chloroplasts are encoded in the nucleus and synthesized in the cytosol as precursor proteins carrying N-terminal transport signals for the 're-import' into the respective target organelle. Most of these transport signals are monospecific, although some of them have dual targeting properties, that is, they are recognized both by mitochondria and by chloroplasts as target organelles. We have identified alpha-MPP2, one of the two isoforms of the substrate binding subunit of mitochondrial processing peptidase of Arabidopsis thaliana, as a novel member of this class of nuclear-encoded organelle proteins. As demonstrated by in organello transport experiments with isolated organelles and by in vivo localization studies employing fluorescent chimeric reporter proteins, the N-terminal region of the alpha-MPP2 precursor comprises transport signals for the import into mitochondria as well as into chloroplasts. Both signals are found within the N-terminal 79 residues of the precursor protein, where they occupy partly separated and partly overlapping regions. Deletion mapping combined with in organello and in vivo protein transport studies demonstrate an unusual architecture of this transport signal, suggesting a composition of three functionally separated domains.

Organelle import of proteins with dual targeting properties into mitochondria and chloroplasts takes place by the general import pathways

“…These experiments fit well to earlier observations [10][11][12] and reconfirm the conclusion that proteins with dual targeting properties do not use distinct import pathways that were specifically developed for this purpose. This observation can be taken as further hint for the hypothesis that dual targeting is an evolutionary remnant in the development of transit peptides of both endosymbiotic organelles.…”

Proteomic analysis of chromoplasts from six crop species reveals insights into chromoplast function and development

“…Approximately 50 plastid proteins have been identified to date as having dual localizations (Carrie et al ., 2009). A recent study reveals that mitochondrial processing peptidase is targeted to both mitochondria and chloroplast (Baudisch and Klosgen, 2012). While the presence of these non-plastid proteins could be most likely due to contamination, the high abundance of them in chromoplast samples may exert effects on chromoplast development in specific plant species.…”

An ambiguous N-terminus drives the dual targeting of an antioxidant protein Thioredoxin peroxidase (TgTPx1/2) to endosymbiotic organelles inToxoplasma gondii

“…Dual targeting of proteins can be achieved through alternative splicing, or the utilization of alternative transcription initiation sites, where the two gene products reach different locations (Danpure, 1995; Small et al, 1998). Additionally, an interesting mechanism observed for dual targeting to the endosymbiotic organelles is that of a single translational product localizing to both chloroplasts and mitochondria (Chew et al, 2003; Karniely & Pines, 2005; Berglund et al, 2009; Baudisch & Klösgen, 2012). In such cases, it has been proposed that ambiguous targeting signals have dual specificities that result in recognition by translocons on both organelles (Chew et al, 2003).…”