GTP HYDROLYSIS IS ESSENTIAL FOR PROTEIN IMPORT INTO THE MITOCHONDRIALMATRIX

Protein import into the innermost compartment of mitochondria (the mat rix) requires a membrane potential (Delta Psi) across the inner membra ne, as well as ATP-dependent interactions with chaperones in the matri x and cytosol, The role of nucleoside triphosphates other than ATP dur ing import into the matrix, however, remains to be determined, Import of urea-denatured precursors does not require cytosolic chaperones, We have there fore used a purified and urea-denatured preprotein in our import assays to bypass the requirement of external ATP, Using this mo dified system, we demonstrate that GTP stimulates protein import into the matrix; the stimulatory effect is directly mediated by GTP hydroly sis and does not result from conversion of GTP to ATP, Both external G TP and matrix ATP are necessary; neither one can substitute for the ot her if efficient import is to be achieved, These results suggest a ''p ush-pull'' mechanism of import, which may be common to other posttrans lational translocation pathways.