Sterol-regulated transport of SREBPs from endoplasmic reticulum to Golgi: Insig renders sorting signal in Scap inaccessible to COPII proteins
- Departments of *Molecular Genetics and
- †Cell Biology, University of Texas Southwestern Medical Center, Dallas, TX 75390
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Edited by Randy Schekman, University of California, Berkeley, CA, and approved February 20, 2007 (received for review January 31, 2007)
Abstract
Two classes of sterols, cholesterol and oxysterols, block export of sterol regulatory element-binding proteins (SREBPs) from the endoplasmic reticulum (ER) to the Golgi by preventing the binding of COPII-coated proteins to a hexapeptide sorting signal (MELADL) in Scap, the SREBP-escort protein. Here, we show that anti-MELADL blocks COPII binding in vitro, and microinjection of Fab anti-MELADL blocks Scap·SREBP movement in cells. Cholesterol and oxysterols block COPII binding to MELADL by binding to different intracellular receptors, cholesterol to Scap and oxysterols to Insig. Cysteine labeling shows that both binding events produce a conformational change near the MELADL sequence, abrogating COPII binding but not anti-MELADL binding. Mutagenesis experiments raise the possibility that the distance of MELADL from the ER membrane is crucial for COPII binding, and we speculate that sterols and Insig block SREBP transport by altering the location of MELADL with respect to the membrane, rendering it inaccessible to COPII proteins.
Footnotes
- ‡To whom correspondence may be addressed. E-mail: mike.brown{at}utsouthwestern.edu or joe.goldstein{at}utsouthwestern.edu
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Author contributions: L.-P.S., J.S., J.L.G., and M.S.B. designed research; L.-P.S. and J.S. performed research; L.-P.S., J.S., J.L.G., and M.S.B. analyzed data; and L.-P.S., J.L.G., and M.S.B. wrote the paper.
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This Feature Article is part of a series identified by the Editorial Board as reporting findings of exceptional significance.
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The authors declare no conflict of interest.
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This article is a PNAS Direct Submission.
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See Commentary on page 6496.
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This article contains supporting information online at www.pnas.org/cgi/content/full/0700907104/DC1.
- Abbreviations:
- SREBP,
- sterol regulatory element-binding protein;
- ER,
- endoplasmic reticulum;
- 25-HC,
- 25-hydroxycholesterol;
- HPCD,
- hydroxypropyl-β-cyclodextrin.
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Freely available online through the PNAS open access option.
- © 2007 by The National Academy of Sciences of the USA
