Cell Biology

Getting Pulled into a Membrane

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Science  19 Oct 2012:
Vol. 338, Issue 6105, pp. 307
DOI: 10.1126/science.338.6105.307-b

When proteins are translocated across the endoplasmic reticulum or bacterial membrane, they pass through a proteinaceous tunnel, the translocon. However, transmembrane proteins need to slip sideways out of the translocon to become embedded in the lipid bilayer. The recognition of transmembrane protein helices by the translocon is a poorly understood process, and competing models have been proposed. One model is a thermodynamic partitioning model for membrane insertion, in which hydrophobic segments in a nascent polypeptide partition between the translocon channel and the surrounding lipid during their passage through the translocon. By using the bacterial SecM or the mammalian Xbp1 translation arrest peptides as in vivo force sensors, Ismail et al. found that a transmembrane helix is subjected to a strong biphasic “pulling force” at the precise moment that it enters the translocon. The pulling force was seen only for peptide segments with hydrophobicity above the threshold for membrane insertion and increased in proportion to the hydrophobicity of the segment. The biphasic force may reflect when the transmembrane segment interacts with and then partitions from the translocon into the membrane.

Nat. Struct. Mol. Biol. 10.1038/nsmb.2376 (2012).

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