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First published online 9 March 2004
doi: 10.1242/jcs.01019

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Active and specific recruitment of a soluble cargo protein for endoplasmic reticulum exit in the absence of functional COPII component Sec24p

Netta Fatal^1,*, Leena Karhinen^1,*, Eija Jokitalo² and Marja Makarow^1,3,

¹ Program in Cellular Biotechnology, University of Helsinki, Viikinkaari 9, 00710 Helsinki, Finland
² Electron Microscopy Unit, Institute of Biotechnology, University of Helsinki, Viikinkaari 9, 00710 Helsinki, Finland
³ Department of Applied Chemistry and Microbiology, University of Helsinki, Viikinkaari 9, 00710 Helsinki, Finland

Author for correspondence (e-mail: marja.makarow{at}helsinki.fi)

Accepted 27 November 2003

Exit of proteins from the yeast endoplasmic reticulum (ER) is thought to occur in vesicles coated by four proteins, Sec13p, Sec31p, Sec23p and Sec24p, which assemble at ER exit sites to form the COPII coat. Sec13p may serve a structural function, whereas Sec24p has been suggested to operate in selection of cargo proteins into COPII vesicles. We showed recently that the soluble glycoprotein Hsp150 exited the ER in the absence of Sec13p function. Here we show that its ER exit did not require functional Sec24p. Hsp150 was secreted to the medium in a sec24-1 mutant at restrictive temperature 37°C, while cell wall invertase and vacuolar carboxypeptidase Y remained in the ER. The determinant guiding Hsp150 to this transport route was mapped to the C-terminal domain of 114 amino acids by deletion analysis, and by an HRP fusion protein-based EM technology adapted here for yeast. This domain actively mediated ER exit of Sec24p-dependent invertase in the absence of Sec24p function. However, the domain was entirely dispensable for ER exit when Sec24p was functional. The Sec24p homolog Sfb2p was shown not to compensate for nonfunctional Sec24p in ER exit of Hsp150. Our data show that a soluble cargo protein, Hsp150, is selected actively and specifically to budding sites lacking normal Sec24p by a signature residing in its C-terminal domain.

Key words: Hsp150, COPI, Yeast, Secretion, Membrane traffic

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