Phospholipid Scramblase 1 Contains a Nonclassical Nuclear Localization Signal with Unique Binding Site in Importin α

Importin β is a major mediator of import into the cell nucleus. Importin β binds cargo molecules either directly or via two types of adapter molecules, importin α, for import of proteins with a classical nuclear localization signal (NLS), or snurportin 1, for import of m3G-capped U snRNPs. Both adapters have an NH2-terminal importin β–binding domain for binding to, and import by, importin β, and both need to be returned to the cytoplasm after having delivered their cargoes to the nucleus. We have shown previously that CAS mediates export of importin α. Here we show that snurportin 1 is exported by CRM1, the receptor for leucine-rich nuclear export signals (NESs). However, the interaction of CRM1 with snurportin 1 differs from that with previously characterized NESs. First, CRM1 binds snurportin 1 50-fold stronger than the Rev protein and 5,000-fold stronger than the minimum Rev activation domain. Second, snurportin 1 interacts with CRM1 not through a short peptide but rather via a large domain that allows regulation of affinity. Strikingly, snurportin 1 has a low affinity for CRM1 when bound to its m3G-capped import substrate, and a high affinity when substrate-free. This mechanism appears crucial for productive import cycles as it can ensure that CRM1 only exports snurportin 1 that has already released its import substrate in the nucleus.

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Phosphorylation adjacent to the nuclear localization signal of human dUTPase abolishes nuclear import: structural and mechanistic insights

Acta Crystallographica Section D Biological Crystallography ◽

10.1107/s0907444913023354 ◽

2013 ◽

Vol 69 (12) ◽

pp. 2495-2505 ◽

Cited By ~ 24

Author(s):

Gergely Róna ◽

Mary Marfori ◽

Máté Borsos ◽

Ildikó Scheer ◽

Enikő Takács ◽

...

Keyword(s):

Cell Cycle ◽

Nuclear Localization ◽

Nuclear Localization Signal ◽

Nucleocytoplasmic Transport ◽

Wild Type ◽

Post Translational Modification ◽

Nuclear Localization Signals ◽

M Phase ◽

Importin Α ◽

Localization Signal

Phosphorylation adjacent to nuclear localization signals (NLSs) is involved in the regulation of nucleocytoplasmic transport. The nuclear isoform of human dUTPase, an enzyme that is essential for genomic integrity, has been shown to be phosphorylated on a serine residue (Ser11) in the vicinity of its nuclear localization signal; however, the effect of this phosphorylation is not yet known. To investigate this issue, an integrated set of structural, molecular and cell biological methods were employed. It is shown that NLS-adjacent phosphorylation of dUTPase occurs during the M phase of the cell cycle. Comparison of the cellular distribution of wild-type dUTPase with those of hyperphosphorylation- and hypophosphorylation-mimicking mutants suggests that phosphorylation at Ser11 leads to the exclusion of dUTPase from the nucleus. Isothermal titration microcalorimetry and additional independent biophysical techniques show that the interaction between dUTPase and importin-α, the karyopherin molecule responsible for `classical' NLS binding, is weakened significantly in the case of the S11E hyperphosphorylation-mimicking mutant. The structures of the importin-α–wild-type and the importin-α–hyperphosphorylation-mimicking dUTPase NLS complexes provide structural insights into the molecular details of this regulation. The data indicate that the post-translational modification of dUTPase during the cell cycle may modulate the nuclear availability of this enzyme.

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Nuclear Localization Signal and Protein Context both Mediate Importin α Specificity of Nuclear Import Substrates

Molecular and Cellular Biology ◽

10.1128/mcb.00708-06 ◽

2006 ◽

Vol 26 (23) ◽

pp. 8697-8709 ◽

Cited By ~ 57

Author(s):

Beate Friedrich ◽

Christina Quensel ◽

Thomas Sommer ◽

Enno Hartmann ◽

Matthias Köhler

Keyword(s):

Nuclear Localization ◽

Nuclear Import ◽

Nuclear Localization Signal ◽

Protein Import ◽

Binding Studies ◽

Vitro Binding ◽

Importin Α ◽

Localization Signal ◽

Experimental Approaches

ABSTRACT The “classical” nuclear protein import pathway depends on importin α and importin β. Importin α binds nuclear localization signal (NLS)-bearing proteins and functions as an adapter to access the importin β-dependent import pathway. In humans, only one importin β is known to interact with importin α, while six α importins have been described. Various experimental approaches provided evidence that several substrates are transported specifically by particular α importins. Whether the NLS is sufficient to mediate importin α specificity is unclear. To address this question, we exchanged the NLSs of two well-characterized import substrates, the seven-bladed propeller protein RCC1, preferentially transported into the nucleus by importin α3, and the less specifically imported substrate nucleoplasmin. In vitro binding studies and nuclear import assays revealed that both NLS and protein context contribute to the specificity of importin α binding and transport.

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Human mRNA Cap Methyltransferase: Alternative Nuclear Localization Signal Motifs Ensure Nuclear Localization Required for Viability

Molecular and Cellular Biology ◽

10.1128/mcb.25.7.2644-2649.2005 ◽

2005 ◽

Vol 25 (7) ◽

pp. 2644-2649 ◽

Cited By ~ 18

Author(s):

Beth Shafer ◽

Chun Chu ◽

Aaron J. Shatkin

Keyword(s):

Nuclear Localization ◽

Nuclear Localization Signal ◽

Nuclear Localization Signals ◽

Truncation Mutant ◽

Capping Enzyme ◽

Importin Α ◽

Localization Signal ◽

Targeting Signals ◽

Sirna Knockdown ◽

Interfering Rna

ABSTRACT A characteristic feature of gene expression in eukaryotes is the addition of a 5′-terminal 7-methylguanine cap (m7GpppN) to nascent pre-mRNAs in the nucleus catalyzed by capping enzyme and cap methyltransferase. Small interfering RNA (siRNA) knockdown of cap methyltransferase in HeLa cells resulted in apoptosis as measured by terminal deoxynucleotidyltransferase-mediated dUTP-tetramethylrhodamine nick end labeling assay, demonstrating the importance of mRNA 5′-end methylation for mammalian cell viability. Nuclear localization of cap methyltransferase is mediated by interaction with importin-α, which facilitates its transport and selective binding to transcripts containing 5′-terminal GpppN. The methyltransferase 96-144 region has been shown to be necessary for importin binding, and N-terminal fusion of this sequence to nonnuclear proteins proved sufficient for nuclear localization. The targeting sequence was narrowed to amino acids 120 to 129, including a required 126KRK. Although full-length methyltransferase (positions 1 to 476) contains the predicted nuclear localization signals 57RKRK, 80KKRK, 103KKRKR, and 194KKKR, mutagenesis studies confirmed functional motifs only at positions 80, 103, and the previously unrecognized 126KRK. All three motifs can act as alternative nu clear targeting signals. Expression of N-truncated cap methyltransferase (120 to 476) restored viability of methyltransferase siRNA knocked-down cells. However, an enzymatically active 144-476 truncation mutant missing the three nuclear localization signals was mostly cytoplasmic and ineffective in preventing siRNA-induced loss of viability.

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