Protein Assembly in Mitochondria

Mitochondria import most of their outer and inner membrane proteins from the cytosol, but also encode a small number of inner membrane proteins in their own genome. As noted above, for proteins 

Three different assembly pathways have been suggested for the imported inner membrane proteins. All three utilize the so-called TOM complex for translocation through the outer membrane. Two of the pathways further make use of the same TIM23 complex in the inner membrane that also handles soluble matrix proteins the third involves both a distinct set of chaperones in the intermembrane space as well as a distinct inner membrane insertion machinery, the TIM22 complex. 

The two TOM-TIM23 pathways—the conservative sorting and stop-transfer pathways—differ in that the former posits a process in which the inner membrane protein is first fully imported into the matrix space and then inserted into the inner membrane from the matrix side using the Oxalp complex (Hell et al., 2001 Stuart and Neupert, 1996). This process is conservative in an evolutionary sense, since the prokaryotic ancestor of present-day mitochondria presumably inserted all their inner membrane proteins from the cytoplasmic (i.e., matrix) side of the membrane. 

Whether the different mitochondrial inner membrane assembly pathways impose different constraints on the final structure of the inner membrane proteins is not known. 

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