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A new role for BiP: closing the aqueous translocon pore during protein integration into the ER membrane

In mammalian cells, most membrane proteins are inserted cotranslationally into the ER membrane at sites termed translocons. Although each translocon forms an aqueous pore, the permeability barrier of the membrane is maintained during integration, even when the otherwise tight ribosome–translocon sea...

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Autores principales: Haigh, Nora G., Johnson, Arthur E.
Formato: Texto
Lenguaje:English
Publicado: The Rockefeller University Press 2002
Materias:
Acceso en línea:https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2199230/
https://www.ncbi.nlm.nih.gov/pubmed/11807091
http://dx.doi.org/10.1083/jcb.200110074
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author Haigh, Nora G.
Johnson, Arthur E.
author_facet Haigh, Nora G.
Johnson, Arthur E.
author_sort Haigh, Nora G.
collection PubMed
description In mammalian cells, most membrane proteins are inserted cotranslationally into the ER membrane at sites termed translocons. Although each translocon forms an aqueous pore, the permeability barrier of the membrane is maintained during integration, even when the otherwise tight ribosome–translocon seal is opened to allow the cytoplasmic domain of a nascent protein to enter the cytosol. To identify the mechanism by which membrane integrity is preserved, nascent chain exposure to each side of the membrane was determined at different stages of integration by collisional quenching of a fluorescent probe in the nascent chain. Comparing integration intermediates prepared with intact, empty, or BiP-loaded microsomes revealed that the lumenal end of the translocon pore is closed by BiP in an ATP-dependent process before the opening of the cytoplasmic ribosome–translocon seal during integration. This BiP function is distinct from its previously identified role in closing ribosome-free, empty translocons because of the presence of the ribosome at the translocon and the nascent membrane protein that extends through the translocon pore and into the lumen during integration. Therefore, BiP is a key component in a sophisticated mechanism that selectively closes the lumenal end of some, but not all, translocons occupied by a nascent chain. By using collisional quenchers of different sizes, the large internal diameter of the ribosome-bound aqueous translocon pore was found to contract when BiP was required to seal the pore during integration. Therefore, closure of the pore involves substantial conformational changes in the translocon that are coupled to a complex sequence of structural rearrangements on both sides of the ER membrane involving the ribosome and BiP.
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spelling pubmed-21992302008-05-01 A new role for BiP: closing the aqueous translocon pore during protein integration into the ER membrane Haigh, Nora G. Johnson, Arthur E. J Cell Biol Article In mammalian cells, most membrane proteins are inserted cotranslationally into the ER membrane at sites termed translocons. Although each translocon forms an aqueous pore, the permeability barrier of the membrane is maintained during integration, even when the otherwise tight ribosome–translocon seal is opened to allow the cytoplasmic domain of a nascent protein to enter the cytosol. To identify the mechanism by which membrane integrity is preserved, nascent chain exposure to each side of the membrane was determined at different stages of integration by collisional quenching of a fluorescent probe in the nascent chain. Comparing integration intermediates prepared with intact, empty, or BiP-loaded microsomes revealed that the lumenal end of the translocon pore is closed by BiP in an ATP-dependent process before the opening of the cytoplasmic ribosome–translocon seal during integration. This BiP function is distinct from its previously identified role in closing ribosome-free, empty translocons because of the presence of the ribosome at the translocon and the nascent membrane protein that extends through the translocon pore and into the lumen during integration. Therefore, BiP is a key component in a sophisticated mechanism that selectively closes the lumenal end of some, but not all, translocons occupied by a nascent chain. By using collisional quenchers of different sizes, the large internal diameter of the ribosome-bound aqueous translocon pore was found to contract when BiP was required to seal the pore during integration. Therefore, closure of the pore involves substantial conformational changes in the translocon that are coupled to a complex sequence of structural rearrangements on both sides of the ER membrane involving the ribosome and BiP. The Rockefeller University Press 2002-01-21 /pmc/articles/PMC2199230/ /pubmed/11807091 http://dx.doi.org/10.1083/jcb.200110074 Text en Copyright © 2002, The Rockefeller University Press This article is distributed under the terms of an Attribution–Noncommercial–Share Alike–No Mirror Sites license for the first six months after the publication date (see http://www.rupress.org/terms). After six months it is available under a Creative Commons License (Attribution–Noncommercial–Share Alike 4.0 Unported license, as described at http://creativecommons.org/licenses/by-nc-sa/4.0/).
spellingShingle Article
Haigh, Nora G.
Johnson, Arthur E.
A new role for BiP: closing the aqueous translocon pore during protein integration into the ER membrane
title A new role for BiP: closing the aqueous translocon pore during protein integration into the ER membrane
title_full A new role for BiP: closing the aqueous translocon pore during protein integration into the ER membrane
title_fullStr A new role for BiP: closing the aqueous translocon pore during protein integration into the ER membrane
title_full_unstemmed A new role for BiP: closing the aqueous translocon pore during protein integration into the ER membrane
title_short A new role for BiP: closing the aqueous translocon pore during protein integration into the ER membrane
title_sort new role for bip: closing the aqueous translocon pore during protein integration into the er membrane
topic Article
url https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2199230/
https://www.ncbi.nlm.nih.gov/pubmed/11807091
http://dx.doi.org/10.1083/jcb.200110074
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