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A more recent version of this article appeared on December 1, 2002
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Submitted on May 30, 2002
Revised on August 5, 2002
Accepted on August 27, 2002
1 Program in Cellular Biotechnology, Institute of Biotechnology, Viikki Biocenter, P.O. Box 56, 00014 University of Helsinki, Finland
* Corresponding author. E-mail address: marja.makarow{at}helsinki.fi.
Sec13p has been thought to be an essential component of the COPII coat, required for exit of proteins from the yeast endoplasmic reticulum (ER). We show here that normal function of Sec13p was not required for ER exit of the Hsp150 glycoprotein. Hsp150 was secreted to the medium under restrictive conditions in a sec13-1 mutant. The COPII components Sec23p and Sec31p, and the GTP/GDP exchange factor Sec12p, were required in functional form for secretion of Hsp150. Hsp150 leaves the ER in the absence of retrograde COPI traffic, and the responsible determinant is a peptide repeated 11 times in the middle of the Hsp150 sequence. Here we localized the sorting determinant for Sec13p-independent ER exit to the C-terminal domain. Sec13p-dependent invertase left the ER in the absence of normal Sec13p function, when fused to the C-terminal domain of Hsp150, demonstrating that this domain contained an active mediator of Sec13p-independent secretion. Thus, Hsp150 harbors two different signatures which regulate its ER exit. Our data show that transport vesicles lacking functional Sec13p can carry out ER-to-Golgi transport, but select only specific cargo protein(s) for ER exit.
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