The Drosophila Mitochondrial Translation Elongation Factor G1 Contains a Nuclear Localization Signal and Inhibits Growth and DPP Signaling

Mutations in the human mitochondrial elongation factor G1 (EF-G1) are recessive lethal and cause death shortly after birth.
We have isolated mutations in iconoclast (ico), which encodes the highly conserved Drosophila orthologue of EF-G1. We find
that EF-G1 is essential during fly development, but its function is not required in every tissue. In contrast to null mutations,
missense mutations exhibit stronger, possibly neomorphic phenotypes that lead to premature death during embryogenesis.
Our experiments show that EF-G1 contains a secondary C-terminal nuclear localization signal. Expression of missense
mutant forms of EF-G1 can accumulate in the nucleus and cause growth and patterning defects and animal lethality. We
find that transgenes that encode mutant human EF-G1 proteins can rescue ico mutants, indicating that the underlying
problem of the human disease is not just the loss of enzymatic activity. Our results are consistent with a model where EF-G1
acts as a retrograde signal from mitochondria to the nucleus to slow down cell proliferation if mitochondrial energy output
is low.