PROTEIN IMPORT INTO SUBSARCOLEMMAL AND INTERMYOFIBRILLAR SKELETAL-MUSCLE MITOCHONDRIA - DIFFERENTIAL IMPORT REGULATION IN DISTINCT SUBCELLULAR REGIONS

To date, no studies have described the import of proteins in mitochond ria obtained from skeletal muscle. In this tissue, mitochondria consis t of the functionally and biochemically distinct intermyofibrillar (IM F) and subsarcolemmal (SS) subfractions, which are localized in specia lized cellular compartments, This mitochondrial heterogeneity in muscl e could be due, in part, to differential rates of protein import, To e valuate this possibility, the import of precursor malate dehydrogenase and ornithine carbamyltransferase proteins was investigated in isolat ed IMF and SS mitochondria in vitro, Import of these was 3-4-fold grea ter in IMF compared with SS mitochondria as a function of time, This c ould account for the higher malate dehydrogenase enzyme activity in IM F mitochondria, Divergent import rates in IMF and SS mitochondria like ly result from a differential reliance on various components of the im port pathway, SS mitochondria possess a greater content of the molecul ar chaperones hsp60 and Grp75, yet import is lower than in IMF mitocho ndria, On the other hand, adriamycin inhibition studies illustrated a greater reliance on acidic phospholipids (i.e. cardiolipin) for the im port process in SS mitochondria, Matrix ATP levels were 3-fold higher in IF mitochondria, but experiments in which ATP depletion was perform ed with atractyloside and oligomycin illustrated a dissociation betwee n import rates and levels of ATP, In contrast, a close relationship wa s found between the rate of ATP production (i.e. mitochondrial respira tion) and protein import, When respiratory rates in IMF and SS mitocho ndria were equalized, import rates in both subfractions were similar, These data indicate that 1) import rates are more closely related to t he rate of ATP production than the steady state ATP level, 2) import i nto IMF and SS mitochondrial subfractions is regulated differently, an d 3) mitochondrial heterogeneity within a cell type can be due to diff erences in the rates of protein import, suggesting that this step is a potentially regulatable event in determining the final mitochondrial phenotype.