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© The Rockefeller University Press, 0021-9525/2003/9/1180-a $5.00
The Journal of Cell Biology, Volume 162, Number 7, 1180-a-1181

Translocon

Transmembrane domains make prolonged contacts with the translocon. Johnson/Elsevier

Translocon proteins are intimately involved with the proteins they import into the ER—including transmembrane portions of the incoming proteins, based on results from Peter McCormick, Arthur Johnson (Texas A&M University, College Station, TX), and colleagues.

Import of membrane proteins into the ER requires a joint effort between the ribosome and the translocon to ensure that each domain of the translocating protein is targeted correctly to either the lumenal or cytoplasmic face of the ER. Transmembrane domains present an additional problem—the hydrophobic portion must move laterally past the translocon into the lipid bilayer. Current models suggest that membrane-spanning domains have very limited contacts with translocon proteins and are instead rapidly surrounded by phospholipids. But the new results show that the translocon is more than a passer-by in this process.

Johnson's group shows that imported transmembrane domains make prolonged contacts with translocon subunits. Photo-crosslinking experiments reveal that one side of a transmembrane helix contacts the translocon protein Sec61, even up until translation is nearly complete. Exact binding sites on translocon proteins varied with the transmembrane sequence, but all sequences tested, including subsequent transmembrane domains, showed prolonged contacts with Sec61.

As in past experiments, the sequences also showed rapid cross-linking with phospholipids. But this does not exclude translocon protein involvement. Rather, says Johnson, "this implies that phospholipids fill in holes in the translocon to avoid a vacuum as it expands to allow the transmembrane domain to move from the pore into the bilayer."

What induces protein release from the translocon is not clear. The affinity of individual transmembrane domain–translocon interactions may dictate the timing of the release. Incoming transmembrane segments may also push out previous domains. If so, orchestrated binding and release may allow the ribosome and translocon to integrate every pair of transmembrane domains correctly in opposite orientations.

Reference:

McCormick, P.J., et al. 2003. Mol. Cell. 12:329–341.[CrossRef][Medline]

Nicole LeBrasseur

lebrasn{at}rockefeller.edu

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This Article Full Text (PDF, 529K) PPT slides of all figures Alert me when this article is cited Services Email this article Similar articles in this journal Alert me to new content in the JCB Download to citation manager Citing Articles Citing Articles via CrossRef Citing Articles via Google Scholar Google Scholar Articles by LeBrasseur, N. Search for Related Content PubMed Articles by LeBrasseur, N. Social Bookmarking                            What's this?