Active maintenance of nuclear actin by importin 9 supports transcription.
Besides its essential and well established role as a component of the cytoskeleton, actin is also present in the cell nucleus, where it has been linked to many processes that control gene expression. For example, nuclear actin regulates the activity of specific transcription factors, associates with all three RNA polymerases, and is a component of many chromatin remodelling complexes. Despite the fact that two export receptors, Crm1 and exportin 6, have been linked to nuclear export of actin, the mechanism by which actin enters the nucleus to elicit these essential functions has not been determined. It is also unclear whether actin is actively exchanged between the nucleus and the cytoplasm, and whether this connection has any functional significance for the cell. By applying a variety of live-cell imaging techniques we revealed that actin constantly shuttles in and out of the nucleus. The fast transport rates, which depend on the availability of actin monomers, suggest an active transport mechanism in both directions. Importantly, we identified importin 9 as the nuclear import factor for actin. Furthermore, our RNAi experiments showed that the active maintenance of nuclear actin levels by importin 9 is required for maximal transcriptional activity. Measurements of nuclear export rates and depletion studies also clarified that nuclear export of actin is mediated by exportin 6, and not by Crm1. These results demonstrate that cytoplasmic and nuclear actin pools are dynamically connected and identify the nuclear import and export mechanisms of actin.
Program in Cell and Molecular Biology, Institute of Biotechnology, University of Helsinki, 00014 Helsinki, Finland.
Joseph
J
Dopie
Kari-Pekka
KP
Skarp
Eeva Kaisa
EK
Rajakylä
Kimmo
K
Tanhuanpää
Maria K
MK
Vartiainen
D051339:Actin Depolymerizing Factors / Q000502:physiology; D000199:Actins / Q000235:genetics / Q000378:metabolism*; D021581:Active Transport, Cell Nucleus / Q000502:physiology*; D000818:Animals; D002460:Cell Line; D003593:Cytoplasm / Q000378:metabolism; D029721:Drosophila Proteins / Q000502:physiology; D004331:Drosophila melanogaster / Q000166:cytology; D017930:Genes, Reporter; D005816:Genetic Complementation Test; D049452:Green Fluorescent Proteins / Q000032:analysis; D006801:Humans; D028884:Karyopherins / Q000037:antagonists & inhibitors / Q000502:physiology*; D051379:Mice; D018613:Microscopy, Confocal; D041681:NIH 3T3 Cells; D038761:Photobleaching; D034622:RNA Interference; D034741:RNA, Small Interfering / Q000494:pharmacology; D018160:Receptors, Cytoplasmic and Nuclear / Q000037:antagonists & inhibitors; D011993:Recombinant Fusion Proteins / Q000378:metabolism; D014158:Transcription, Genetic / Q000502:physiology*; D028961:beta Karyopherins / Q000502:physiology*; D020931:ran GTP-Binding Protein / Q000502:physiology; D000097606:Exportin 1 Protein
D051339:Actin Depolymerizing Factors; D000199:Actins; D029721:Drosophila Proteins; D049452:Green Fluorescent Proteins; D028884:Karyopherins; D034741:RNA, Small Interfering; D018160:Receptors, Cytoplasmic and Nuclear; D011993:Recombinant Fusion Proteins; D028961:beta Karyopherins; D020931:ran GTP-Binding Protein; D000097606:Exportin 1 Protein; C571728:exportin 6, mouse; C571727:importin 9, mouse