Channels in the mitochondrial outer membrane exchange metabolites, ions, and proteins with the rest of the cell. Kruger et al. identify several new types of channel and suggest that the outer mitochondrial membrane is a more selective molecular sieve with a greater variety of channel-forming proteins than previously appreciated.

Two peroxisomal targeting signals, PTS1 and PTS2, recognized by cytosolic receptors Pex5 and cooperating Pex7/Pex18, direct folded proteins to the peroxisomal matrix. A pore consisting of the PTS1 receptor Pex5 and the docking protein Pex14 imports PTS1 proteins. We identified a distinct PTS2-specific pore, which contains the PTS2 co-receptor Pex18 and the Pex14/Pex17-docking complex as major constituents. The estimated maximal pore size of ∼ 4.7 nm is large enough to allow import of folded PTS2 proteins. PTS2 cargo proteins modulate complex gating, open probability, and subconductance states of the pore. While the PTS1 channel is transiently activated by arriving receptor-cargo complexes, the reconstituted PTS2 channel is constitutively present in an open state. However, the cargo-loaded PTS2 channel is largely impermeable to solutes and ions. Our results demonstrate that import of PTS1 and PTS2 proteins does not converge at the peroxisomal membrane as previously anticipated but is performed by distinct pores.

A novel technique is described which comprises a base-specific DNA duplex formation at a lipid bilayer-H(2) O-phase boundary layer. Two different probes of oligonucleotides both carrying a double-tailed lipid at the 5'-terminus were incorporated into stable artificial lipid bilayers separating two compartments (cis/trans-channel) of an optically transparent microfluidic sample carrier with perfusion capabilities. Both the cis- and trans-channels are filled with saline buffer. Injection of a cyanine-5-labeled target DNA sequence, which is complementary to only one of the oligonucleotide probes, into the cis-channel, followed by a thorough perfusion, leads to an immobilization of the labeled complementary oligonucleotide on the membrane as detected by single-molecule fluorescence spectroscopy and microscopy. In the case of fluorescent but non-complementary DNA sequences, no immobilized fluorescent oligonucleotide duplex could be detected on the membrane. This clearly verifies a specific duplex formation at the membrane interface.

2'-Deoxyinosine (1) and thymidine (7) were N-alkylated with geranyl and farnesyl moieties. These hydrophobic derivatives, 3a and 3b, and 9a and 9b, respectively, represent the first synthetic biomimetic nucleoterpenes and were subsequently 5'-protected and converted into the corresponding 3'-O-phosphoramidites, 5a and 5b and 11a and 11b, respectively. The latter were used to prepare a series of lipophilized oligonucleotide dodecamers, a part of which were additionally labelled with indocarbocyanine fluorescent dyes (Cy3 or Cy5), 18-23. The insertion of the lipooligonucleotides into, as well as duplex formation at artificial lipid bilayers was studied by single-molecule fluorescence spectroscopy and fluorescence microscopy.