CONTRACTILE ACTIVITY-INDUCED ADAPTATIONS IN THE MITOCHONDRIAL PROTEINIMPORT SYSTEM

We previously demonstrated that subsarcolemmal (SS) and intermyofibril lar (IMF) mitochondrial subfractions import proteins at different rate s. This study was undertaken to investigate 1) whether protein import is altered by chronic contractile activity, which induces mitochondria l biogenesis, and 2) whether these two subfractions adapt similarly. U sing electrical stimulation (10 Hz, 3 h/day for 7 and 14 days) to indu ce contractile activity, we observed that malate dehydrogenase import into the matrix of the SS and IMF mitochondia isolated from stimulated muscle was significantly increased by 1.4- to 1.7-fold, although the pattern of increase differed for each subfraction. This acceleration o f import may be mitochondrial compartment specific, since the import o f Bcl-2 into the outer membrane was not affected. Contractile activity also modified the mitochondrial content of proteins comprising the im port machinery, as evident from increases in the levels of the intrami tochondrial chaperone mtHSP70 as well as the outer membrane import rec eptor Tom20 in SS and IMF mitochondria. Addition of cytosol isolated f rom stimulated or control muscles to the import reaction resulted in s imilar twofold increases in the ability of mitochondria to import mala te dehydrogenase, despite elevations in the concentration of mitochond rial import-stimulating factor within the cytosol of chronically stimu lated muscle. These results suggest that chronic contractile activity modifies the extra-and intramitochondrial environments in a fashion th at favors the acceleration of precursor protein import into the matrix of the organelle. This increase in protein import is likely an import ant adaptation in the overall process of mitochondrial biogenesis.