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Expression of inflammatory genes is determined in part by post-transcriptional regulation of mRNA metabolism but how stimulus- and transcript-dependent nuclear export influence is poorly understood. Here, we report a novel pathway in which LPS/TLR4 engagement promotes nuclear localization of IRAK2 to facilitate nuclear export of a specific subset of inflammation-related mRNAs for translation in murine macrophages. IRAK2 kinase activity is required for LPS-induced RanBP2-mediated IRAK2 sumoylation and subsequent nuclear translocation. Array analysis showed that an SRSF1-binding motif is enriched in mRNAs dependent on IRAK2 for nuclear export. Nuclear IRAK2 phosphorylates SRSF1 to reduce its binding to target mRNAs, which promotes the RNA binding of the nuclear export adaptor ALYREF and nuclear export receptor Nxf1 loading for the export of the mRNAs. In summary, LPS activates a nuclear function of IRAK2 that facilitates the assembly of nuclear export machinery to export selected inflammatory mRNAs to the cytoplasm for translation.

The RanGTPase activating protein RanGAP1 has essential functions in both nucleocytoplasmic transport and mitosis. In interphase, a significant fraction of vertebrate SUMO1-modified RanGAP1 forms a stable complex with the nucleoporin RanBP2/Nup358 at nuclear pore complexes. RanBP2 not only acts in the RanGTPase cycle but also is a SUMO1 E3 ligase. Here, we show that RanGAP1 is phosphorylated on residues T409, S428, and S442. Phosphorylation occurs before nuclear envelope breakdown and is maintained throughout mitosis. Nocodazole arrest leads to quantitative phosphorylation. The M-phase kinase cyclin B/Cdk1 phosphorylates RanGAP1 efficiently in vitro, and T409 phosphorylation correlates with nuclear accumulation of cyclin B1 in vivo. We find that phosphorylated RanGAP1 remains associated with RanBP2/Nup358 and the SUMO E2-conjugating enzyme Ubc9 in mitosis, hence mitotic phosphorylation may have functional consequences for the RanGTPase cycle and/or for RanBP2-dependent sumoylation.

Small ubiquitin-related modifier (SUMO) proteins are reversibly coupled to numerous intracellular targets and modulate their interactions, localization, activity or stability. Recent advances in the SUMO field have uncovered the first SUMO E3 ligases and point to a complex family of isopeptidases. SUMO has been linked to many different pathways, including nucleocytoplasmic transport. Modifying enzymes and an isopeptidase have been detected at nuclear pore complexes. In addition, studies in yeast suggest a requirement of SUMO conjugation for nuclear protein import, and specific SUMO targets depend on modification for nuclear import or export.

Research has been conducted on the Wnt-responsive transcription factor lymphoid enhancer factor 1 (LEF1) which activates transcription in association with beta-catenin and represses transcription in association with Groucho. The additional regulatory mechanisms of the LEF1 function have been investigated via the identified as an interaction partner of LEF1 protein which inhibits the activated signal transducers and activators of transcription and protein inhibitor of activated STAT, and the details are reported.

SUMO-1, a novel peptide of 101 amino acids, has been found to be covalently linked to the mammalian Ran GTPase-activating protein RanGAP1 in an adenosine triphosphate-dependent reaction. SUMO-1 exhibits a homology to ubiquitin, a 76-residue protein found in eukaryotes. Data indicate that the polypeptide is involved in protein targeting and not degradation. At least three additional intranuclear proteins are modified with SUMO-1 or with a protein closely resembling it.

Microtubule depolymerases of the kinesin‐13 family play important roles in various cellular processes and are frequently overexpressed in different cancer types. Despite the importance of their correct abundance, remarkably little is known about how their levels are regulated in cells. Using comprehensive screening on protein microarrays, we identified 161 candidate substrates of the multi‐subunit ubiquitin E3 ligase SCFFbxw5, including the kinesin‐13 member Kif2c/MCAK. In vitro reconstitution assays demonstrate that MCAK and its closely related orthologs Kif2a and Kif2b become efficiently polyubiquitylated by neddylated SCFFbxw5 and Cdc34, without requiring preceding modifications. In cells, SCFFbxw5 targets MCAK for proteasomal degradation predominantly during G2. While this seems largely dispensable for mitotic progression, loss of Fbxw5 leads to increased MCAK levels at basal bodies and impairs ciliogenesis in the following G1/G0, which can be rescued by concomitant knockdown of MCAK, Kif2a or Kif2b. We thus propose a novel regulatory event of ciliogenesis that begins already within the G2 phase of the preceding cell cycle. SYNOPSIS: Despite their importance in normal cellular physiology and tumorigenesis, regulation of microtubule depolymerases remains little understood. Here, the kinesin‐13 family member MCAK is found to be ubiquitylated and degraded via SCFFbxw5 to facilitate ciliogenesis. SCFFbxw5 modifies kinesin‐13 proteins with K48‐linked polyubiquitin chains.SCFFbxw5‐dependent polyubiquitylation targets MCAK for degradation during G2 phase, while post‐mitotic degradation is driven by the APC/C ligase.Defects in MCAK ubiquitylation and degradation cause its accumulation at basal bodies during the subsequent G1/G0 phase, impairing ciliogenesis. [ABSTRACT FROM AUTHOR]

GTPase Ran is a low molecular weight cytosolic factor activated by the protein Ran. Light and electron immunolocalization showed that Ran localizes at the cytoplasmic surface of the nuclear pore complex during nuclear protein import in the presence of nonhydrolyzable analogs of GTP. Binding studies showed that Ran-GTP binds with a pore complex protein. Blot overlay techniques identified this protein to be the 358-kD protein RanBP2. It is suggested that RAN-GTP binding to the nuclear pore complex occurs at or near the region where RanBP2 localizes.

Is SUMO unique in the way it regulates proteins? No. SUMOylation is one of many types of modification that can change and regulate the function of a protein. Such modifications [...]