Biochemical and biophysical characterization of the protein import pathway of the mitochondrion

Over the course of the evolution between the cell and the mitochondrion, an elaborate set of translocons has been developed. This set of translocons was developed to facilitate the transport of nuclear-coded proteins into mitochondria, because the mitochondria) genome only encodes a small percentage of the proteins destined to reside within the mitochondrion. The vast majority of proteins bound for the matrix have a cleavable N-terminal targeting sequence and import is facilitated via the import pathway which involves TOM complex (translocase of the outer membrane) and the TIM23 complex (translocase of the inner membrane). However some proteins have targeting information that does not reside at the N-terminal region and does not get cleaved, rather this information is located within the mature form of the protein. Some of these proteins generally reside in the inner membrane and use the TIM22 translocon for entry. The unique feature of the TIM22 translocon is that it has two completely soluble components that play the role of chaperones for this translocation pathway, the Tim9p-Tim10p and the Tim8p-Timl3p complexes. These two chaperones form 70 kDa complexes in the intermembrane space and facilitate transport of hydrophobic proteins from the TOM complex to the TIM22 complex. The mechanisms by which these chaperones initially form, and then bind to and release their substrate has been elusive. In addition, the region or motif that these chaperones recognize on their substrates is also unknown. This study describes the function of these complexes, the Tim8p-Tim13p more directly, in import as well as the mechanism and specificity of the chaperone interacting with its substrate.