Mutations in TIC100 impair and repair chloroplast protein import and impact retrograde signalling

Chloroplast biogenesis requires synthesis of proteins in the nucleocytoplasm and the chloroplast itself. Nucleus-encoded chloroplast proteins are imported via multiprotein translocons in the organelles envelope membranes. Controversy exists around whether a 1 MDa complex comprising TIC20, TIC100 and other proteins constitutes the inner membrane TIC translocon. The Arabidopsis 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 the gun1 mutation. 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. One sentence summaryComplementary mutations in TIC100 of the chloroplast inner envelope membrane cause reductions or corrective improvements in chloroplast protein import, and highlight a signalling role.