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Published 29 March 2004. doi:10.1083/jcb.200312079
The Rockefeller University Press, 0021-9525 $8.00
JCB, Volume 164, Number 7, 997-1007
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Article

The organization of engaged and quiescent translocons in the endoplasmic reticulum of mammalian cells



Erik L. Snapp, Gretchen A. Reinhart, Brigitte A. Bogert, Jennifer Lippincott-Schwartz, and Ramanujan S. Hegde

Cell Biology and Metabolism Branch, National Institute of Child Health and Human Development, National Institutes of Health, Bethesda, MD 20892

Address correspondence to R.S. Hegde, Cell Biology and Metabolism Branch, National Institute of Child Health and Human Development, National Institutes of Health, 18 Library Dr., Bldg. 18, Rm. 101, Bethesda, MD 20892. Tel.: (301) 496-4855. Fax: (301) 402-0078. email: hegder{at}mail.nih.gov

Protein translocons of the mammalian endoplasmic reticulum are composed of numerous functional components whose organization during different stages of the transport cycle in vivo remains poorly understood. We have developed generally applicable methods based on fluorescence resonance energy transfer (FRET) to probe the relative proximities of endogenously expressed translocon components in cells. Examination of substrate-engaged translocons revealed oligomeric assemblies of the Sec61 complex that were associated to varying degrees with other essential components including the signal recognition particle receptor TRAM and the TRAP complex. Remarkably, these components not only remained assembled but also had a similar, yet distinguishable, organization both during and after nascent chain translocation. The persistence of preassembled and complete translocons between successive rounds of transport may facilitate highly efficient translocation in vivo despite temporal constraints imposed by ongoing translation and a crowded cellular environment.

Key Words: FRET; TRAP; protein translocation; Sec61; TRAM

The online version of this article includes supplemental material.

Abbreviations used in this paper: CNX, calnexin; FRET, fluorescence resonance energy transfer; SR, SRP receptor; SRP, signal recognition particle.


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