Cell Biology

Assembling the Assemblers

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Science  27 Apr 2001:
Vol. 292, Issue 5517, pp. 603
DOI: 10.1126/science.292.5517.603c

Within eukaryotic cells, the mitochondria are semiautonomous organelles that are bounded by a pair of membranes and that generate chemical energy for the cell in the form of ATP. Mitochondria do contain their own genome, which is essential for their maintenance, but most of their proteins are encoded by nuclear genes and need to be imported from the cytoplasm into the organelle.

Model et al. have examined how the protein import channel, which resides in the mitochondrial outer membrane, is itself put in place. The core protein of the import machinery is TOM (translocase of the outer membrane) 40, which forms a passageway across the outer membrane. In combination with TOM20 (a receptor for newly synthesized mitochondrial proteins) and three smaller components (TOM5, TOM6, and TOM7), TOM40 assembles into a 400-kilodalton complex in the outer membrane. Surprisingly, during its own import, TOM40 passes right across the outer membrane into the intermembrane space where it first associates with TOM5, then reinserts into the outer membrane where other components are added to create the mature import complex. One explanation of this peculiar behavior is that the intramembraneous portion of the import complex adopts an inflexible β-barrel tertiary structure, which cannot open sideways to allow a TOM40 protein to exit while in transit. — SMH

Naure Struct. Biol.8, 361 (2001).

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