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

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 (Deltahcd_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 (Cin(cif1) cells) are genetically and phenotypically distinct from the previously characterized Cin(Deltahcd_cnx1) cells, which spontaneously appear in the presence of the Deltahcd_Cnx1p mutant. Moreover, cif1(+) is not required for the induction or maintenance of the Cin(Deltahcd_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 Cin(cif1) state, thus suggesting an unexpected interaction between the vital cellular role of calnexin and a function of the nucleolus.