Question: To aid your studies of protein import into mitochondria, you treat cells with cycloheximide, whic…

To aid your studies of protein import into mitochondria, you
treat cells with cycloheximide, which blocks ribosome movement
along mRNA. When you examine these cells in the electron
microscope, you are surprised to find cytosolic ribosomes attached
top the outside of the mitochondria. By contrast, you have never
seen attached ribosomes in the absence of cycloheximide. To
investigate this phenomenon further, you prepare mitochondria from
cells that have been treated with cycloheximide and then extract
the mRNA that is bound to the ribosomes associated with the
mitochondria. You translate this mRNA in vitro and compare the
protein products with similarly translated mRNA from the cytosol.
The results are clear-cut: the mitochondria-associated ribosomes
are translating mRNAs that encode mitochondrial proteins. You are
astounded!!! Here, clearly visible in the electron micrographs,
seems to be proof that protein import into mitochondria occurs
during translation. How might you reconcile this result with the
prevailing view that mitochondrial proteins are imported after they
have been synthesized and released from ribosomes?