Biogenesis of the mitochondrial phosphate carrier

The mitochondrial phosphate carrier (PiC) is a member of the family
of inner-membrane carrier proteins which are generally synthesized
without a cleavable presequence. Surprisingly, the cDNA sequences
of bovine and rat PiC suggested the existence of an amino-terminal
extension sequence in the precursor of PiC. By expressing PiC in
vitro, we found that PiC is indeed synthesized as a larger
precursor. This precursor was imported and proteolytically
processed by mitochondria, whereby the correct amino-terminus of
the mature protein was generated. Import of PiC showed the
characteristics of mitochondrial protein uptake, such as dependence
on ATP and a membrane potential and involvement of contact sites
between mitochondrial outer and inner membranes. The precursor
imported in vitro was correctly assembled into the functional form,
demonstrating that the authentic import and assembly pathway of PiC
was reconstituted when starting with the presequence-carrying
precursor. These results are discussed in connection with the
recently postulated role of PiC as an import receptor located in
the outer membrane.