Sorting of D-lactate dehydrogenase to the inner membrane of mitochondria. Analysis of topogenic signal and energetic requirements.

D-Lactate dehydrogenase (D-LD) is located in the inner membrane of mitochondria. It spans the membrane once in an Nin-Cout orientation with the bulk of the protein residing as a folded domain in the intermembrane space. D-LD is synthesized as a precursor with an N-terminal cleavable presequence and is imported into the mitochondria in a Deltapsi-dependent, but mt-Hsp70-independent manner. Upon import in vitro D-LD folds in the intermembrane space to attain a conformation indistinguishable from endogenous D-LD. Sorting of D-LD to the inner membrane is directed by a composite topogenic signal consisting of the hydrophobic transmembrane segment and a cluster of charged amino acids C-terminal to it. We propose a model for the mode of operation of the sorting signal of D-LD. This model also accounts for the driving force of translocation across the outer membrane, in the apparent absence of mt-Hsp70-dependent assisted import and involves the folding of the D-LD in the intermembrane space.