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Protein transport

The doors to organelles

Nature volume 395, pages 439–440 (1998)Cite this article

Mitochondria import most of their proteins from the surrounding cytosol. During this process, proteins destined for import are first selected from the thousands of cytosolic proteins and are then transported into or across the outer membrane of the organelle. Finally, many of them must be sorted to their correct destination inside. Reports by Hill et al.1 (page 516 of this issue) and Künkele et al.2 (in Cell) now provide exciting information on the molecular architecture of the channels that transport proteins through the outer membranes of mitochondria.

Hill et al.1 now provide direct and convincing evidence for this hypothesis. They overproduced yeast mitochondrial Tom40 in Escherichia coli and purified it by denaturation followed by renaturation. They then incorporated the pure Tom40 asymmetrically into liposomes or planar phospholipid bilayers, and showed that the reconstituted protein forms a hydrophilic, cation-selective transmembrane channel. The channel is partly blocked by a peptide representing a functional mitochondrial targeting sequence, but only if the peptide is added to the side of the artificial membrane that corresponds to the cytosolic face of the mitochondrial outer membrane. Calculations based on the channel conductance indicate a pore diameter of 12 to 26 Å, depending on the assumed resistivity on the channel — when the authors calibrated the channel with differently sized cations they found a pore diameter of 22 Å. This value agrees with observations that the channel is big enough to allow the passage of precursors that are attached to double-stranded DNA6. Furthermore, measurements of the Tom channel with calibrated gold particles yield a value of greater than 22 Å but less than 26 Å (A. Matouschek, personal communication).

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Figure 1: The protein-transport channels in the outer membrane of mitochondria (Tom) and chloroplasts (Toc).

References

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  1. the Department of Biochemistry, Biozentrum, Basel University, Klingelbergstrasse 70, CH-4056, Basel, Switzerland

    Gottfried Schatz

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  1. Gottfried Schatz
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Schatz, G. The doors to organelles. Nature 395, 439–440 (1998). https://doi.org/10.1038/26620

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