Mitochondrial protein import and assembly dynamics in response to chronic contractile activity in skeletal muscle of young and aged animals

Mitochondrial protein import adapts to perturbations in cellular energy status by altering the rate at which nuclear‐encoded proteins are translocated into mitochondria. The purposes of this study were to determine whether the import and assembly of precursor proteins are similar in muscle from young and aged F344XBN animals, and to assess the effect of chronic contractile activity (CCA) on the import pathway in subsarcolemmal (SS) mitochondria. Import of Tom40 into the outer membrane increased (p<0.05) over time in both young and aged animals, but was not different between the two groups. However, assembly of the multisubunit TOM complex containing Tom40, as assessed by Blue Native‐PAGE, was 2.2‐fold greater (p<0.05) in aged, when compared to young animals. Following 7 days of CCA (3hr/day), Tom40 assembly into the TOM complex was accelerated by 75%, compared to non‐stimulated muscle in young animals. This increase due to CCA was only 65% in aged animals (p<0.05). CCA also resulted in an 85% increase in the import of the matrix protein OCT in SS mitochondria from young animals, while only a 28% increase in OCT import was observed in aged animals (p<0.05). Our findings suggest that aged muscle has a greater capacity to assemble Tom40 into a functional TOM complex, when compared to young animals. CCA can further accelerate the rate of import and assembly in both groups, however this effect is attenuated with age.