Academic journal article Genetics

The Fission Yeast Schizosaccharomyces Pombe Has Two Importin-[Alpha] Proteins, Imp1p and Cut15p, Which Have Common and Unique Functions in Nucleocytoplasmic Transport and Cell Cycle Progression

Academic journal article Genetics

The Fission Yeast Schizosaccharomyces Pombe Has Two Importin-[Alpha] Proteins, Imp1p and Cut15p, Which Have Common and Unique Functions in Nucleocytoplasmic Transport and Cell Cycle Progression

Article excerpt

ABSTRACT

The nuclear import of classical nuclear localization signal-containing proteins depends on importin-α transport receptors. In budding yeast there is a single importin-α gene and in higher eukaryotes there are multiple importin-α-like genes, but in fission yeast there are two: the previously characterized cut15 and the more recently identified imp1. Like other importin-α family members, Imp1p supports nuclear protein import in vitro. In contrast to cut15, imp1 is not essential for viability, but imp1Δ mutant cells exhibit a telophase delay and mild temperature-sensitive lethality. Differences in the cellular functions that depend on Imp1p and Cut15p indicate that they each have unique physiological roles. They also have common roles because the imp1Δ and the cut15-85 temperature-sensitive mutations are synthetically lethal; overexpression of cut15 partially suppresses the temperature sensitivity, but not the mitotic delay in imp1Δ cells; and overexpression of imp1 partially suppresses the mitotic defect in cut15-85 cells but not the loss of viability. Both Imp1p and Cut15p are required for the efficient nuclear import of both an SV40 nuclear localization signal-containing reporter protein and the Pap1p component of the stress response MAP kinase pathway. Imp1p and Cut15p are essential for efficient nuclear protein import in S. pombe.

NUCLEOCYTOPLASMIC transport is a process specific to eukaryotes in which the chromosomes are physically separated from the cytoplasm by the nuclear envelope (NE). In all eukaryotes, the receptormediated transport of nuclear proteins across the NE from their site of synthesis in the cytoplasm is essential for all nuclear processes. The precise tissue-specific and temporal regulation of nuclear protein import is also critical in the regulation of cell cycle progression and in developmental and signal transduction pathways (reviewed in KAFFMAN and O'SHEA 1999).

Proteins are targeted to the nucleus by an NLS (nuclear focalization signal). There are two types of classical NLSs, both of which must bind to an importin-a adaptor for transport to the nucleus: the mono-partite NLS that consists of 4 or more basic amino acids preceded by a helix-breaking residue and the bipartite NLS that has two short stretches of basic arnino acids separated by a 9- to 12-amino-acid spacer (reviewed in IZAURRALDE and ADAM 1998). Both the mono-partite SV40 and the bipartite nucleoplasmin NLS are competent to direct cargo proteins to the nucleus in fission yeast, budding yeast, and other organisms (reviewed in YOSHIDA and SAZER 2004).

The importin-β family of transport receptors, also called karyopherins, or more specifically, importins or exportins, carries cargoes both into and out of the nucleus. Some import cargoes bind directly to an importin-β receptor, while others interact with an importin-a adaptor, which associates with an importin-β receptor (GoRLiCH and KUTAY 1999). The importin-β subunit of both of these types of transport complex targets them to the NE by binding to proteins at the nuclear pore complex (reviewed in GORLICH and KUTAY 1999).

There are multiple importin-α proteins in metazoan organisms and they have been categorized on the basis of amino-acid sequence comparisons (MALIK et al. 1997; MASON et al. 2002). Three subfamilies, whose members have different expression patterns, different functions, and/or different cargo-binding specificities, have been identified in plants and animals (KOHLER et al. 1999; TALCOTT and MOORE 2000; GELES and ADAM 2001; GELES et al. 2002; MASON et al. 2002). In Drosophila, importin-α isoforms have nonoverlapping functions: the gametogenesis defects of a null mutation in importin-a2 can be rescued by importin-αι or -a3 in male flies, but only by importin-a2 in females (MASON et al. 2002). The in vivo cargoes of the importin-αι, -a2, and -a3 families remain largely unknown. In budding yeast there is a single importin-α-like gene, SKPl, but in fission yeast there are two, impl and cut! …

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