Mutations in the chloroplast inner envelope protein TIC100 impair and repair chloroplast protein import and impact retrograde signaling

Abstract

Chloroplast biogenesis requires synthesis of proteins in the nucleocytoplasm and the chloroplast itself. Nucleus-encoded chloroplast proteins are imported via multiprotein translocons in the organelle’s envelope membranes. Controversy exists around whether a 1 MDa complex comprising TIC20, TIC100 and other proteins constitutes the inner membrane TIC translocon. The Arabidopsis thaliana cue8 virescent mutant is broadly defective in plastid development. We identify CUE8 as TIC100. The tic100cue8 mutant accumulates reduced levels of 1 MDa complex components and exhibits reduced import of two nucleus-encoded chloroplast proteins of different import profiles. A search for suppressors of tic100cue8 identified a second mutation within the same gene, tic100soh1, which rescues the visible, 1 MDa complex-subunit abundance, and chloroplast protein import phenotypes. tic100soh1 retains but rapidly exits virescence and rescues the synthetic lethality of tic100cue8 when retrograde signalling is impaired by a mutation in the GENOMES UNCOUPLED 1 gene. Alongside the strong virescence, changes in RNA editing and the presence of unimported precursor proteins show that a strong signalling response is triggered when TIC100 function is altered. Our results are consistent with a role for TIC100, and by extension the 1 MDa complex, in the chloroplast import of photosynthetic and non-photosynthetic proteins, a process which initiates retrograde signalling.

Golgi‐localized MORN1 promotes lipid droplet abundance and enhances tolerance to multiple stresses in Arabidopsis

“…Similar tethering and transfer facilitating activities have been observed in other MORN motif‐containing proteins. JPH proteins in muscle and neuronal cells tether the ER to the PM for lipid transfer and calcium dynamics between the ER and the PM (Woo et al, 2016), and TIC100 on the chloroplast inner envelope is critical for chloroplast protein import (Loudya et al, 2022). Because of the low expression of MORN1‐GFP in transgenic plants, all localization studies were carried out by transient expression in Arabidopsis protoplasts and N. benthamiana leaves.…”

Understanding protein import in diverse non-green plastids

“…Furthermore, high levels of import are observed for a subset of proteins when TIC56 or YCF1 are inhibited (Köhler et al, 2015;Bölter and Soll, 2017). Mutant tic100 Arabidopsis plastids import less than one-third of the protein of wild type plants, and the abundance of 1-MDa translocation complex are reduced by more than one-half, however (Loudya et al, 2022). TIC21 is suggested to be an essential translocon component (Teng et al, 2006;Kikuchi et al, 2009), while it has also been characterized an iron transporter and is phylogenetically related to cyanobacterial permeases (Duy et al, 2007); later studies indicated that it does not co-purify with the TOC/TIC translocation complex (Kikuchi et al, 2013;Nakai, 2015a).…”

Chloroplast proteostasis: A story of birth, life, and death

“…Indeed, PhANG expression in gun1 is only higher than that of the lincomycin/norflurazon-treated wild type; it is still much lower than that of the untreated wild type or gun1 mutants. Thus, this model cannot be used to explain the reduced expression of PhANGs in ppi2 , tic100 cue8 , or any other virescent mutants (with import defects) in comparison with the healthy wild type ( Kakizaki et al., 2009 ; Loudya et al., 2022 ). The reduced protein import in the ppi2 or tic100 cue8 mutant represses the biogenesis of chloroplasts, thus triggering downregulation of PhANG expression, further indicating that chloroplasts with disturbed proteostasis emit a retrograde signal to tune nuclear gene expression for rebalancing proteostasis.…”