Biosynthesis and mitochondrial processing of the beta subunit of propionyl coenzyme A carboxylase from rat liver.

Propionyl-CoA carboxylase (ADP-forming) (EC 6.4.1.3), an oligomer of nonidentical subunits (alpha 4 beta 4), has been localized to the mitochondrial matrix. As a first step in examining this enzyme's biogenesis, we have investigated in vitro the cell-free, rat liver RNA-directed synthesis of the beta subunit, and its post-translational transport and processing by rat liver mitochondria. The beta subunit is synthesized as a precursor approximately 7,500 daltons larger than its mature mitochondrial counterpart. The extension segment, comprising approximately 60 amino acids, is located at the NH2 terminus of the precursor. Intact mitochondria translocate the precursor across both mitochondrial membranes, and a protease localized to the mitochondrial matrix cleaves the precursor to a polypeptide identical in size and peptide composition to the mature beta subunit.