Interaction between mitochondrial precursor proteins and cytosolic soluble domains of mitochondrial import receptors, Tom20 and Tom70, measured by surface plasmon resonance.

We studied the interaction between mitochondrial precursor proteins and postulated mitochondrial surface receptor proteins, Tom20 and Tom70, by using a methodology of surface plasmon resonance. For these studies, import-competent mitochondrial precursor proteins, pCOXIV-DHFR and pSu9-DHFR, and cytosolic domains of the two receptor proteins were separately expressed in and purified from E. coli cells as a soluble form. By measuring surface plasmon resonance, both of the purified precursor proteins were found to specifically bind to either of the cytosolic domains of import receptors immobilized on a sensor chip. On the other hand, import-incompetent SynB2-DHFR and DHFR itself were shown to possess little or no binding abilities to the sensor chip, respectively. Using this system, we could demonstrate that the proposed carboxy-terminal acidic bristle domain of Tom20 is not essential for the precursor binding. Chemical modification of the acidic amino acid residues of either cytosolic domain on the sensor chip partially inhibited the binding of pSu9-DHFR, whereas the binding of pCOXIV-DHFR was almost unaffected. These results suggest that distinct set of amino acid residues of the receptor proteins might be responsible for the binding of different precursor proteins.