Figure 1.

Oxa1p is required for the export of N-terminal tails of imported proteins from the mitochondrial matrix. Isolated wild-type (wt) and pet ts1402 mitochondria were preincubated for 10 min in import buffer either at 0°C or at the nonpermissive temperature of 37°C. After cooling on ice, reticulocyte lysate containing radiolabeled precursors pSu9(1–112)DHFR (A) and pSu9(1–66)pCoxII(1–74)DHFR (B) together with NADH (4 mM) were added. Import reactions were performed at 25°C for 30 min. Samples were divided and either mock-treated or subjected to proteinase K (PK) treatment under nonswelling or swelling conditions, as indicated. Samples were analyzed by SDS/PAGE and autoradiography and were quantified by densitometry. Swelling efficiency was determined as described under Materials and Methods and is given as percent mitochondria that had been converted to mitoplasts. m, MPP-processed form of imported species protease inaccessible in the mitoplasts; f, specific fragment of the imported protein generated by the degradation of intermembrane space (IMS)-exposed exported segments of the protein by proteinase K under swelling conditions. Both m and f forms are depicted here and are further described in text.