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Vol. 13, Issue 12, 4130-4140, December 2002
Program in Cellular Biotechnology, Institute of Biotechnology,
Viikki Biocenter, 00014 University of Helsinki, Finland
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 herein 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. Herein, 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 that 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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