title: Nuclear import of ho endonuclease utilizes two nuclear localization signals and four importins of the ribosomal import system.
abstract: Activity of Ho, the yeast mating switch endonuclease, is restricted to a narrow time window of the cell cycle. Ho is unstable and despite being a nuclear protein is exported to the cytoplasm for proteasomal degradation. We report here the molecular basis for the highly efficient nuclear import of Ho and the relation between its short half-life and passage through the nucleus. The Ho nuclear import machinery is functionally redundant, being based on two bipartite nuclear localization signals, recognized by four importins of the ribosomal import system. Ho degradation is regulated by the DNA damage response and Ho retained in the cytoplasm is stabilized, implying that Ho acquires its crucial degradation signals in the nucleus. Ho arose by domestication of a fungal VMA1 intein. A comparison of the primary sequences of Ho and fungal VMA1 inteins shows that the Ho nuclear localization signals are highly conserved in all Ho proteins, but are absent from VMA1 inteins. Thus adoption of a highly efficient import strategy occurred very early in the evolution of Ho. This may have been a crucial factor in establishment of homothallism in yeast, and a key event in the rise of the Saccharomyces sensu stricto.
authors.affiliation: Department of Life Sciences, Ben Gurion University of the Negev, P. O. Box 653, 84105 Beersheba, Israel.
authors.forename: Anya
authors.initials: A
authors.lastname: Bakhrat
authors.forename: Keren
authors.initials: K
authors.lastname: Baranes
authors.forename: Oleg
authors.initials: O
authors.lastname: Krichevsky
authors.forename: Inna
authors.initials: I
authors.lastname: Rom
authors.forename: Gabriel
authors.initials: G
authors.lastname: Schlenstedt
authors.forename: Shmuel
authors.initials: S
authors.lastname: Pietrokovski
authors.forename: Dina
authors.initials: D
authors.lastname: Raveh
mesh_terms: D021581:Active Transport, Cell Nucleus; D000595:Amino Acid Sequence; D001692:Biological Transport; D003593:Cytoplasm / Q000378:metabolism; D015252:Deoxyribonucleases, Type II Site-Specific / Q000502:physiology*; D047668:Inteins; D028884:Karyopherins / Q000378:metabolism; D008969:Molecular Sequence Data; D009154:Mutation; D019913:Nuclear Localization Signals; D010802:Phylogeny; D010957:Plasmids / Q000378:metabolism; D012270:Ribosomes / Q000378:metabolism*; D012441:Saccharomyces cerevisiae / Q000378:metabolism; D029701:Saccharomyces cerevisiae Proteins
chemical_list: D028884:Karyopherins; D019913:Nuclear Localization Signals; D029701:Saccharomyces cerevisiae Proteins; C530845:HO protein, S cerevisiae; C037725:SCEI protein, S cerevisiae; D015252:Deoxyribonucleases, Type II Site-Specific