Porin Propinquity

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Science  26 Mar 2004:
Vol. 303, Issue 5666, pp. 1947
DOI: 10.1126/science.303.5666.1947c

Recent successes in membrane protein stucture determination have advanced our understanding of how small hydrophilic entities (such as K+, glycerol, and H2O) can be transported passively across hydrophobic biological membranes. How larger and less readily movable entities such as proteins are transported is still a bit fuzzy, although as usual, bacterial systems are likely to offer the first glimpses.

Oomen et al. describe the β-barrel structure of the C-terminal translocator domain of the autotransporter NalP from Neisseria meningitidis; an α-helical linker segment (which joins the N-terminal functional domain to the translocator) neatly fills the pore, which has an inner diameter of approximately 1 nm. Autotransporters encode and secrete virulence-related proteins (for example, a protease that attacks IgA molecules), and thus transit of the functional domain across the bacterial outer membrane via the β barrel would seem a plausible scenario.

Yet the answer isn't quite so simple. Threading the functional domain, N-terminal end first, into and through the barrel would require both a targeting signal, which has not yet been identified, and an energy source analogous to the ribosome-powered translocation of proteins across the inner membrane. On the other hand, using the energy from folding to drive transport would mean pulling the C-terminal end of the functional domain through first (it is the C-terminal portion that folds first), but there isn't enough room in the pore for a helical hairpin, so the polypeptide would have to be dragged across in a fully unfolded and extended form. A third possibility, akin to the entry mechanism for colicin E3 proposed by Kurisu et al., is that the translocator domain serves primarily to bring the functional domain close to an outer membrane porin, which serves as the actual protein transporter. — GJC

EMBO J. 10.1038/sj.emboj.7600148 (2004); Nature Struct.Biol. 10, 948 (2003).

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