The localization of Tic20 proteins in Arabidopsis thaliana is not restricted to the inner envelope membrane of chloroplasts

Abstract

Tic20 is a central, membrane-embedded component of the precursor protein translocon of the inner envelope of chloroplasts (TIC). In Arabidopsis thaliana, four different isoforms of Tic20 exist. They are annotated as atTic20-I, -II, -IV and -V and form two distinct phylogenetic subfamilies in embryophyta. Consistent with atTic20-I being the only essential isoform for chloroplast development, we show that the protein is exclusively targeted to the chloroplasts inner envelope. The same result is observed for atTic20-II. In contrast, atTic20-V is localized in thylakoids and atTic20-IV dually localizes to chloroplasts and mitochondria. These results together with the previously established expression profiles explain the recently described phenotypes of Tic20 knockout plants and point towards a functional diversification of these proteins within the family. For all Tic20 proteins a 4-helix topology is proposed irrespective of the targeted membrane, which in part could be confirmed in vivo by application of a self-assembling GFP-based topology approach. By the same approach we show that the inner envelope localized Tic20 proteins expose their C-termini to the chloroplast stroma. This localization would be consistent with the positive inside rule considering a stromal translocation intermediate as discussed.

Chloroplast β-Barrel Proteins Are Assembled into the Mitochondrial Outer Membrane in a Process That Depends on the TOM and TOB Complexes

“…Only if the two proteins, to which the GFP fragments are fused, are located in the same cellular compartment can a GFP signal be observed (34). As expected, if both fragments were in the cytosol or the IMS of chloroplasts was a GFP signal observed in the corresponding compartment (Fig.…”

Biogenic membranes of the chloroplast in Chlamydomonas reinhardtii

“…Generality of this model is supported by the identification of a PSII biogenesis compartment in the cyanobacterium Synechocystis sp. PCC 6803 and a GFP-tagged Tic20 paralogue which was seen to be localized to specific regions of the chloroplast envelope in A. thaliana (28,29). The effect of light on the relocalization of the TIC and TOC import machinery to lobe junctions might be relevant to the light stimulation of chloroplast protein import which has been observed in C. reinhardtii and vascular plants (24,30).…”

Stress-Induced Chloroplast Degradation in Arabidopsis Is Regulated via a Process Independent of Autophagy and Senescence-Associated Vacuoles

“…The results showed that the CCVs that are close to, but not associated with, broken chloroplasts also could be labeled by antibodies raised against PsbO1, a subunit of the photosystem II (PSII) complex localized in the thylakoid membrane (Supplemental Figure 4B). Moreover, CCVs also contained Tic20-II, a protein from chloroplast inner envelope membranes (Machettira et al, 2011) (Supplemental Figure 5B). These results suggested that CCVs were generated from chloroplast membranes that were disrupted by CV.…”