YEAST MITOCHONDRIA LACKING THE PHOSPHATE CARRIER P32 ARE BLOCKED IN PHOSPHATE-TRANSPORT BUT CAN IMPORT PREPROTEINS AFTER REGENERATION OF A MEMBRANE-POTENTIAL/

Two different functions have been proposed for the phosphate carrier p rotein/p32 of Saccharomyces cerevisiae mitochondria: transport of phos phate and requirement For import of precursor proteins into mitochondr ia. We characterized a yeast mutant lacking the gene for the phosphate carrier/p32 and found both a block in the import of phosphate and a s trong reduction in the import of preproteins transported to the mitoch ondrial inner membrane and matrix, Binding of preproteins to the surfa ce of mutant mitochondria and import of outer membrane proteins were n ot inhibited, indicating that the inhibition of protein import occurre d after the recognition step at the outer membrane. The membrane poten tial across the inner membrane of the mutant mitochondria was strongly reduced, Restoration of the membrane potential restored preprotein im port but did not affect the block of phosphate transport of tile mutan t mitochondria. We conclude that the inhibition of protein import into mitochondria lacking the phosphate carrier/p32 is indirectly caused b y a reduction of the mitochondrial membrane potential (Delta gamma), a nd we propose a model that the reduction of Delta psi is due to the de fective phosphate import, suggesting that phosphate transport is the p rimary function of the phosphate carrier/p32.