Contractile activity-induced adaptations in the mitochondrial protein import system

We previouslydemonstrated that subsarcolemmal (SS) and intermyofibrillar (IMF)mitochondrial subfractions import proteins at different rates. Thisstudy was undertaken to investigate1) whether protein import is alteredby chronic contractile activity, which induces mitochondrialbiogenesis, and 2) whether these two subfractions adapt similarly. Using electrical stimulation (10 Hz, 3 h/day for 7 and 14 days) to induce contractile activity, we observedthat malate dehydrogenase import into the matrix of the SS and IMFmitochondia isolated from stimulated muscle was significantly increasedby 1.4- to 1.7-fold, although the pattern of increase differed for eachsubfraction. This acceleration of import may be mitochondrialcompartment specific, since the import of Bcl-2 into the outer membranewas not affected. Contractile activity also modified the mitochondrialcontent of proteins comprising the import machinery, as evident fromincreases in the levels of the intramitochondrial chaperone mtHSP70 aswell as the outer membrane import receptor Tom20 in SS and IMFmitochondria. Addition of cytosol isolated from stimulated or controlmuscles to the import reaction resulted in similar twofold increases inthe ability of mitochondria to import malate dehydrogenase, despiteelevations in the concentration of mitochondrial import-stimulatingfactor within the cytosol of chronically stimulated muscle. Theseresults suggest that chronic contractile activity modifies the extra- and intramitochondrial environments in a fashion that favors the acceleration of precursor protein import into the matrix of the organelle. This increase in protein import is likely an important adaptation in the overall process of mitochondrial biogenesis.