Protein translocations across mitochondria membranes play critical roles in mitochondria biogenesis. The protein transports from the cell cytosol to the mitochondria are carried out by the translocase of the outer membrane (TOM) complex and the translocase of the inner membrane (TIM) complex. (1) In the TOM complex, Tom70 functions as the receptor for mitochondria precursors with internal targeting signals. In our Tom70 crystal structure, the C-terminal domain of Tom70 forms a large pocket which may represent the binding site for mitochondrial precursor and the N-terminal domain of Tom70 may function to gate the pocket. Interestingly, the gating of the precursor-binding pocket of Tom70 is regulated by Hsp70/Hsp90 binding. The crystal structure of Tom70-Hsp70/Hsp90 complex indicates that the C-terminal EEVD motifs of Hsp70/Hsp90 can maintain Tom70 in the open conformation for receiving mitochondrial precursor. To fully understand the mechanism how Tom70 interacts with its peptide substrate under the regulation of Hsp70/Hsp90, we have identified a peptide substrate P70-8 for Tom70-Hsp70 complex by phage display library screening. The crystal structure of Tom70-Hsp70 EEVD motif-peptide substrate complex will illustrate the mechanism how Tom70 functions as a receptor for the molecular chaperone-bound mitochondrial precursor in the TOM translocon. Structure-based mutagenesis studies will be performed to confirm our hypothesis. (2) In the TIM23 translocon, Tim50 functions as a receptor to guide the precursor with the N-terminal presequence to the inner membrane protein channel Tim23 for translocation. Tim50IMS may interact with the presequence. Tim50IMS can also interact with Tim23IMS to deliver the precursors to the transmembrane channel formed by the C-terminal domain of Tim23. Our crystal structure of Tim50IMS indicated a protruding -hairpin may represent the binding site for Tim23. Close to this -hairpin, Tim50 contains a large groove that may represent the binding site for the presequence. We intend to determine the crystal structures of Tim50IMS-presequence complex, Tim50IMS- Tim23IMS complex and Tim50IMS-Tim23IMS-presequence complex. (3) Tim23 represents the major component in TIM23 translocon and it forms the essential transmembrane channel in the mitochondrial inner membrane. To reveal the mechanism how this important membrane protein transports mitochondrial precursors, we propose to determine the crystal structure of Tim23. Tim23 has been known to be difficult to express using a number of systems. In preliminary data, we have developed a crystallization chaperone for yeast Tim23IMS using phage display library screening. We have successfully expressed Tim23 complexed with the crystallization chaperone using the self-cleaving 2A peptide in Pichia system. The recombinant Tim23 is functional as shown by electrophysiological analysis using planar lipid bilayer system.
Protein translocations across mitochondria membranes play critical roles in mitochondria biogenesis. The long-term goal of this proposal is to carry out structural and functional studies on TOM and TIM complexes to uncover the basic mechanisms how the translocons facilitate the protein translocations across the mitochondria outer and inner membranes. The proposed research may have broad impacts on human aging, cancer biology and diabetes.
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