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Published online 28 December 2004. doi:10.1083/jcb.200408106
The Rockefeller University Press, 0021-9525 $8.00
JCB, Volume 168, Number 1, 79-88
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Article

 Specialized membrane-localized chaperones prevent aggregation of polytopic proteins in the ER

Jhansi Kota and Per O. Ljungdahl

Ludwig Institute for Cancer Research, S-17177 Stockholm, Sweden

Correspondence to Per O. Ljungdahl: plju{at}licr.ki.se

The integral endoplasmic reticulum (ER) membrane protein Shr3p is required for proper plasma membrane localization of amino acid permeases (AAPs) in yeast. In the absence of Shr3p AAPs are uniquely retained in the ER with each of their twelve membrane-spanning segments correctly inserted in the membrane. Here, we show that the membrane domain of Shr3p specifically prevents AAPs from aggregating, and thus, plays a critical role in assisting AAPs to fold and correctly attain tertiary structures required for ER exit. Also, we show that the integral ER proteins, Gsf2p, Pho86p, and Chs7p, function similarly to Shr3p. In cells individually lacking one of these components only their cognate substrates, hexose transporters, phosphate transporters, and chitin synthase-III, respectively, aggregate and consequently fail to exit the ER membrane. These findings indicate that polytopic membrane proteins depend on specialized membrane-localized chaperones to prevent inappropriate interactions between membrane-spanning segments as they insert and fold in the lipid bilayer of the ER membrane.

Abbreviations used in this paper: AAP, amino acid permease; DM, dodecyl-ß-D-maltopyranoside; Dpm1p, dolichol phosphate mannose synthase; DSP, Dithiobis[succinimidyl propionate]; LacY, lactose permease; PM, plasma membrane; SANPAH, N-succinimidyl 6-[4'-azido 2'-nitrophenylamino]hexanoate; SC, synthetic complex; TMS, transmembrane segments.


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