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

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 tic100(cue8) 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 tic100(cue8) identified a second mutation within the same gene, tic100(soh1), which rescues the visible, 1 MDa complex-subunit abundance, and chloroplast protein import phenotypes. tic100(soh1) retains but rapidly exits virescence and rescues the synthetic lethality of tic100(cue8) when retrograde signaling 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 signaling 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 nonphotosynthetic proteins, a process which initiates retrograde signaling. Complementary mutations in TIC100 of the chloroplast inner envelope membrane cause reductions or corrective improvements in chloroplast protein import, and highlight a signaling role.

Automated summary

Chloroplast biogenesis requires the synthesis of proteins in both the nucleus and the chloroplast itself. A 1-MDa complex, including TIC20, TIC100, and other proteins, is believed to be the inner membrane TIC translocon for importing nucleus-encoded chloroplast proteins. This study investigated the role of TIC100 in chloroplast development and protein import. Mutations in TIC100 resulted in reduced levels of the 1-MDa complex and decreased import of nucleus-encoded chloroplast proteins. A second mutation within TIC100 rescued the phenotypes of the first mutation and highlighted the importance of TIC100 in retrograde signaling. These findings suggest that TIC100 and the 1-MDa complex play crucial roles in chloroplast protein import and retrograde signaling.