title: Normal telomere length maintenance in Saccharomyces cerevisiae requires nuclear import of the ever shorter telomeres 1 (Est1) protein via the importin alpha pathway.
abstract: The Est1 (ever shorter telomeres 1) protein is an essential component of yeast telomerase, a ribonucleoprotein complex that restores the repetitive sequences at chromosome ends (telomeres) that would otherwise be lost during DNA replication. Previous work has shown that the telomerase RNA component (TLC1) transits through the cytoplasm during telomerase biogenesis, but mechanisms of protein import have not been addressed. Here we identify three nuclear localization sequences (NLSs) in Est1p. Mutation of the most N-terminal NLS in the context of full-length Est1p reduces Est1p nuclear localization and causes telomere shortening-phenotypes that are rescued by fusion with the NLS from the simian virus 40 (SV40) large-T antigen. In contrast to that of the TLC1 RNA, Est1p nuclear import is facilitated by Srp1p, the yeast homolog of importin α. The reduction in telomere length observed at the semipermissive temperature in a srp1 mutant strain is rescued by increased Est1p expression, consistent with a defect in Est1p nuclear import. These studies suggest that at least two nuclear import pathways are required to achieve normal telomere length homeostasis in yeast.
authors.affiliation: Biological Sciences Department, Vanderbilt University, Nashville, Tennessee, USA.
authors.forename: Charlene
authors.initials: C
authors.lastname: Hawkins
authors.affiliation: Biological Sciences Department, Vanderbilt University, Nashville, Tennessee, USA Katherine.friedman@vanderbilt.edu.
authors.forename: Katherine L
authors.initials: KL
authors.lastname: Friedman
mesh_terms: D021581:Active Transport, Cell Nucleus; D000595:Amino Acid Sequence; D002467:Cell Nucleus / Q000378:metabolism*; D008969:Molecular Sequence Data; D019913:Nuclear Localization Signals; D029543:Nucleocytoplasmic Transport Proteins / Q000378:metabolism; D016601:RNA-Binding Proteins / Q000378:metabolism; D012441:Saccharomyces cerevisiae / Q000235:genetics* / Q000378:metabolism; D029701:Saccharomyces cerevisiae Proteins / Q000737:chemistry / Q000378:metabolism*; D019098:Telomerase / Q000737:chemistry / Q000378:metabolism*; D059505:Telomere Homeostasis; D028961:beta Karyopherins / Q000378:metabolism
chemical_list: C423162:KAP122 protein, S cerevisiae; C110033:MTR10 protein, S cerevisiae; D019913:Nuclear Localization Signals; D029543:Nucleocytoplasmic Transport Proteins; D016601:RNA-Binding Proteins; D029701:Saccharomyces cerevisiae Proteins; D028961:beta Karyopherins; C099024:EST1 protein, S cerevisiae; D019098:Telomerase