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Published online April 7, 2008
doi:10.1083/jcb.200710093
The Journal of Cell Biology, Vol. 181, No. 1, 105-118
The Rockefeller University Press, 0021-9525 $30.00
© 2008 Ronchi et al.
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Article

 Transmembrane domain–dependent partitioning of membrane proteins within the endoplasmic reticulum

Paolo Ronchi1,2, Sara Colombo1,2, Maura Francolini1,2, and Nica Borgese1,2,3

1 Consiglio Nazionale delle Ricerche, Institute of Neuroscience, Cellular and Molecular Pharmacology, University of Milan, 20129 Milan, Italy
2 Department of Medical Pharmacology, University of Milan, 20129 Milan, Italy
3 Faculty of Pharmacy, University of Catanzaro Magna Graecia, 88021 Roccelletta di Borgia (CZ), Italy

Correspondence to Nica Borgese: n.borgese{at}in.cnr.it

The length and hydrophobicity of the transmembrane domain (TMD) play an important role in the sorting of membrane proteins within the secretory pathway; however, the relative contributions of protein–protein and protein–lipid interactions to this phenomenon are currently not understood. To investigate the mechanism of TMD-dependent sorting, we used the following two C tail–anchored fluorescent proteins (FPs), which differ only in TMD length: FP-17, which is anchored to the endoplasmic reticulum (ER) membrane by 17 uncharged residues, and FP-22, which is driven to the plasma membrane by its 22-residue-long TMD. Before export of FP-22, the two constructs, although freely diffusible, were seen to distribute differently between ER tubules and sheets. Analyses in temperature-blocked cells revealed that FP-17 is excluded from ER exit sites, whereas FP-22 is recruited to them, although it remains freely exchangeable with the surrounding reticulum. Thus, physicochemical features of the TMD influence sorting of membrane proteins both within the ER and at the ER–Golgi boundary by simple receptor-independent mechanisms based on partitioning.

Abbreviations used in this paper: COP, coat protein complex; cyt, cytochrome; D, diffusion coefficient; ERES, ER exit site; ERGIC, endoplasmic reticulum–Golgi intermediate compartment; FI, fluorescence intensity; FP, fluorescent protein; IC, intermediate compartment; Mf, mobile fraction; Rtn, reticulon; TA, tail anchored; TMD, transmembrane domain; VSVG, ts045 vesicular stomatitis virus glycoprotein.


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