[38] Synthesis and intracellular transport of mitochondrial matrix proteins in rat liver: studies in vivo and in vitro

This chapter describes some of the methods employed to study the uptake of precursor proteins into the matrix fraction of liver mitochondria in rats and frogs. The chapter focuses on two enzymes, carbamoyl-phosphate synthetase (CPS) and ornithine carbamoyltransferase (OCT). CPS and OCT catalyze the first and second steps, respectively, of the urea cycle. The CPS is particularly abundant, accounting for about 30% of matrix protein, or about 4–5% of total liver protein, whereas OCT represents only about 1% of mitochondrial matrix protein. The precursor polypeptides are synthesized by free polyribosomes, then pass rapidly through the cytosol, cross both mitochondrial membranes in a posttranslational manner, and are finally deposited in the matrix. Endoproteolytic processing takes place at some point either coincident with or immediately following transmembrane uptake of the precursors by mitochondria. A comparison of the kinetics of all of these events for CPS and OCT showed a remarkable similarity between the two proteins.