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J. Biol. Chem., Vol. 283, Issue 26, 18040-18047, June 27, 2008

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Differential Sorting and Golgi Export Requirements for Raft-associated and Raft-independent Apical Proteins along the Biosynthetic Pathway^*

Christopher J. Guerriero¹², Yumei Lai¹, and Ora A. Weisz³

From the Departments of Medicine and Cell Biology and Physiology, University of Pittsburgh School of Medicine, Pittsburgh, Pennsylvania 15261

Sorting signals for apically destined proteins are highly diverse and can be present within the luminal, membrane-associated, and cytoplasmic domains of these proteins. A subset of apical proteins partition into detergent-resistant membranes, and the association of these proteins with glycolipid-enriched microdomains or lipid rafts may be important for their proper targeting. Recently, we observed that raft-associated and raft-independent apical proteins take different routes to the apical surface of polarized Madin-Darby canine kidney cells (Cresawn, K. O., Potter, B. A., Oztan, A., Guerriero, C. J., Ihrke, G., Goldenring, J. R., Apodaca, G., and Weisz, O. A. (2007) EMBO J. 26, 3737–3748). Here we reconstituted in vitro the export of raft-associated and raft-independent markers staged intracellularly at 19 °C. Surprisingly, whereas release of the raft-associated protein influenza hemagglutinin was dependent on the addition of an ATP-regenerating system and cytosol, release of a yellow fluorescent protein (YFP)-tagged raft-independent protein (the 75-kDa neurotrophin receptor; YFP-p75) was efficient even in the absence of these constituents. Subsequent studies suggested that YFP-p75 is released from the trans-Golgi network in fragile tubules that do not withstand isolation procedures. Moreover, immunofluorescence analysis revealed that hemagglutinin and YFP-p75 segregate into distinct subdomains of the Golgi complex at 19 °C. Our data suggest that raft-associated and raft-independent proteins accumulate at distinct intracellular sites upon low temperature staging, and that upon warming, they exit these compartments in transport carriers that have very different membrane characteristics and morphologies.

Received for publication, March 14, 2008 , and in revised form, April 16, 2008.

^* This work was supported, in whole or in part, by National Institutes of Health Grant DK054407 (to O. A. W.). The costs of publication of this article were defrayed in part by the payment of page charges. This article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.

¹ These authors contributed equally to this work.

² Supported by an American Heart Association predoctoral fellowship.

³ To whom correspondence should be addressed: Renal-Electrolyte Division, University of Pittsburgh School of Medicine, 3550 Terrace St., Pittsburgh PA 15261. E-mail: weisz{at}pitt.edu.

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