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First published online 7 April 2009
doi: 10.1242/jcs.040949

Journal of Cell Science 122, 1342-1351 (2009)
Published by The Company of Biologists 2009
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Research Article

A nucleolar protein allows viability in the absence of the essential ER-residing molecular chaperone calnexin

Pascale B. Beauregard1, Renée Guérin1, Cynthia Turcotte1, Susan Lindquist2 and Luis A. Rokeach1,*

1 Department of Biochemistry, Université de Montréal, C.P. 6128, succursale Centre-ville, Montréal, Québec H3C 3J7, Canada
2 Whitehead Institute for Biomedical Research and Howard Hughes Medical Institute, 9 Cambridge Center, Cambridge, MA 02142, USA

* Author for correspondence (e-mail: luis.rokeach{at}umontreal.ca)

Accepted 5 January 2009

In fission yeast, the ER-residing molecular chaperone calnexin is normally essential for viability. However, a specific mutant of calnexin that is devoid of chaperone function ({Delta}hcd_Cnx1p) induces an epigenetic state that allows growth of Schizosaccharomyces pombe without calnexin. This calnexin-independent (Cin) state was previously shown to be mediated via a non-chromosomal element exhibiting some prion-like features. Here, we report the identification of a gene whose overexpression induces the appearance of stable Cin cells. This gene, here named cif1+ for calnexin-independence factor 1, encodes an uncharacterized nucleolar protein. The Cin cells arising from cif1+ overexpression (Cincif1 cells) are genetically and phenotypically distinct from the previously characterized Cin{Delta}hcd_cnx1 cells, which spontaneously appear in the presence of the {Delta}hcd_Cnx1p mutant. Moreover, cif1+ is not required for the induction or maintenance of the Cin{Delta}hcd_cnx1 state. These observations argue for different pathways of induction and/or maintenance of the state of calnexin independence. Nucleolar localization of Cif1p is required to induce the Cincif1 state, thus suggesting an unexpected interaction between the vital cellular role of calnexin and a function of the nucleolus.

Key words: Endoplasmic reticulum, Epigentics, Fission yeast, Molecular chaperone, Nucleolus, S. pombe


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