The IQ-compete assay for measuring mitochondrial protein import efficiencies in living yeast cells.

Most mitochondrial proteins are synthesized in the cytosol and imported into the organelle. Here, we describe a novel Import and de-Quenching Competition (IQ-compete) assay which monitors the import efficiency of model proteins by fluorescence in living cells. For this method, the sequence of the tobacco etch virus (TEV) protease is fused to a mitochondrial precursor and coexpressed with a cytosolic reporter which becomes fluorescent upon TEV cleavage. Thus, inefficient import of the fusion protein leads to a fluorescent signal. With the IQ-compete assay, the import efficiency of proteins can be reliably analyzed in fluorescence readers, by flow cytometry, by microscopy, and by western blotting. We are convinced that the IQ-compete assay will be a powerful strategy for many different applications. Impact statement This article describes a novel method to monitor the mitochondrial import efficiency for a given protein in living yeast cells. With this IQ-compete assay, protein import efficiencies can be quantified by fluorescent microscopy, flow cytometry, fluorescence spectrometry or western blotting.