Most mitochondrial proteins are synthesized in the cytosol and imported into mitochondria by the outer membrane translocator, TOM complex. The TOM complex consists of the channel-forming -barrel protein Tom40 and the six other subunits, each containing single -helical transmembrane segments. Insertion and assembly processes of these subunits, especially Tom22, remain unclear. We identified Por1 (porin) and cytosolic Hsp70 (Ssa1) as interaction partners of Tom22 upon its assembly into the TOM complex by in vivo photocrosslinking (Shiota et al., unpublished). Porin is an abundant -barrel protein in the outer membrane and functions as a channel for small molecules, but then how does por1 act on the assembly of Tom22? We isolated Por1-depleted mitochondria from yeast, and analyzed the import of RI-labeled Tom22 in vitro. The assembly of newly-synthesized Tom22 into the 440 kDa trimeric TOM complex was accelerated, while Tom40 assembly was marginally retarded, in the absence of Por1. A previous study revealed that the trimeric 440 kDa TOM complex containing Tom40 and Tom22 undergoes the dynamic equilibrium with the dimeric TOM complex containing Tom40, but not Tom22 (Shiota et al., Science (2016)). Since Por1 appears to stabilize unassembled free Tom22 molecules, Por1 likely modulates the dynamic equilibrium of the TOM complex between those with and without Tom22. Physiological significance of this Por1-mediated modulation of the Tom22 assembly into the TOM complex is now under investigation.
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