PURIFIED PRESEQUENCE BINDING-FACTOR (PBF) FORMS AN IMPORT-COMPETENT COMPLEX WITH A PURIFIED MITOCHONDRIAL PRECURSOR PROTEIN

In vitro mitochondrial import of the purified precursor form (pOTC) of rat ornithine carbamoyltransferase (OTC) is stimulated by a cytosolic factor(s) contained in rabbit reticulocyte lysate. A protein factor that binds to pOTC but not to mature OTC and was named presequence binding factor or PBF, was purified 91000-fold from the lysate by affinity chromatography using pOTC-bound Sepharose, DEAE- 5PW HPLC and sucrose gradient centrifugation. The purified PBF migrated as a single polypeptide of 50000 daltons on SDS-PAGE. On sucrose gradients, urea-denatured pOTC sedimented to the bottom, whereas PBF sedimented with an s(20,w) value of 5.5S. When pOTC and PBF were centrifuged together, both polypeptides sedimented as a complex of 7.1S. Formation of the pOTC-PBF complex was inhibited by micromolar concentrations of the synthetic presequence of pOTC and those of other mitochondrial precursor proteins. The purified PBF markedly stimulated the import of purified of in vitro synthesized pOTC into the mitochondria. PBF-stimulated pOTC import was further enhanced by a 70 kd heat shock protein (hsp70) purified from yeast; the hsp70 alone had little effect. Thus, PBF binds to the presequence portion of the precursors and may hold them in a transport-competent form in cooperation with hsp70.