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Annotated Information


Fig-1.Rice importina1 supported nuclear import of NLS-GFP in vertebratein vitroassay system.from ref[1]

Os01g0253300 encodes importin subunit alpha-1a,a member of importin family, which functions a essential role in nuclear protein import by specifically and directly binding to substrates containing either a simple or bipartite NLS motif[2].In vivo assay suggests that importin alpha-1a can replace vertebrate importin a in the mediation of nuclear import of NLS substrates, implying that rice importin a1 functions as a NLS receptor in the process of nuclear import of proteins(Fig-1)[1],which indicates the importin family has a conserved transportation function both in plants and mammals.The importin alpha-1a also has a function in virus infection by interact with mungbean yellow mosaic virus capsid protein[3].


Fig-2.Transcript analyses of rice importin.
Fig-3.An increase of importin-atranscript levels during dark-adaptation of light-grown green leaves.From ref[4]

Transcript analyses of rice importin alpha-1a by Chang-Jie Jiang et al shows importin alpha-1a is highly expressed in callus, followed by root and etiolated leaf and lowly expressed in green leaf(Fig-2)[2].The expression level of importin alpha-1a is greatly increased after darkt treatment in green leaf[Fig-3][4] .However it has a constitute expression pattern in non-photosynthetic tissues[Fig-4][3].Together,it indicates a complex regulation of the importin alpha-1a transcription.

Fig4 constitue exprssion.jpg

Fig-4.Importin-atranscript levels in different rice tissues.(A) RNA was prepared from various tissues, roots (Root), mature leaf blades (M-Leaf), flag leaf blades (F-Leaf), stems (Stem) and suspension-cultured cells (Callus), and the transcript levels for importin-α in their tissues were examined by RNA blot analysis. Mature leaves, flag leaves and stems of field-grown plants were collected at noon on a sunny day in July. (B) The effect of illumination on the transcript levels for importin-α was examined in roots and suspension-cultured cells. Roots of water-cultured seedlings and suspension-cultured cells were exposed to white light (white fluorescent lamps; 90 μmol m−2 s−1) for 24 h (L), kept in complete darkness for 24 h (D) or kept in darkness for 24 h and then re-illuminated for 3 h (+3hL).From ref [3]

Protein structure

1.The structure of rice importin alpha-1a comprises 10 ARM repeats (green, cartoon representation) and two NLS-like sequences from the N-terminal IBB domain(shown in yellow stick )[FIG-5][5]. Fig5 crystal structure of importin.jpg

Fig-5:crystal structure of importin alpha-1a.From ref[5]

2.The crystal structures of their complexes with rice importin alpha-1a show that they bind to the minor NLS binding site[Fig6A]. By contrast, the crystal structures of their complexes with mouse (Mus musculus) importin alpha-1ashow preferential binding to the major NLS binding site[Fig6B]. The results reveal the molecular basis of a number of features of the classical nuclear transport pathway specific to plants[5].

Fig 6 structure difference.jpg

Fig-6:Differential Binding of Plant-Specific NLSs to rImpa1a and mImpa.[5]


The phylogenetic tree of importin alpha shows that the rice importin alpha-1a protein is the most distant member from various organisms[Fig-7][6].we could find another EST from Arabidopsis (accession number F15465), which has about 60% identity with #61L at the levelof both nucleotide and amino acid sequences. It might be interpreted that importin alpha-1a homologous to rice #61L(Refere to importin alpha-1a) is also present in Arabidopsis, and that the #61L protein and the homologue are functionally differentiated from the known importin alpha-1a.

Fig 7.jpg

Fig-7:(Phylogenetic tree of importinK. The tree was constructed by the UPGMA method using the GENETYX-MAC 7.3 software(Software Development Co., Tokyo) with default parameters. The accession number of the putative open reading frame T10M13.16 which is predicted ) From[6]

Knowledge Extension

Rice encodes many kinds of importin proteins such as importin α1a/1b,importinβand importin α1a is the most studied gene in rice with its crystal structure complex being resolved[5][7]. Importin α1a functions as another importin α protein by form a complex with importin βand NLS-containing substrate.As shown in Figure -8, a single round of importin α-mediated import can be divided into six steps: (i) formation of a ternary complex in the cytoplasm; (ii) importin β-mediated binding of the ternary complex to docking sites at the periphery of the NPC; (iii) importin β-mediated translocation through the NPC; (iv) dissociation of the ternary complex, triggered, in part, by the binding of the small nuclear GTPase Ran–GTP to importin β; (v) recycling of importin α to the cytoplasm bound to the exportin CAS–Ran–GTP; and (vi) disassembly of the export complex and release of free importin α to the cytoplasm. Disassembly is induced by the Ran–GAP-induced hydrolysis of GTP by Ran. Ran–GDP and CAS are recycled back to the nucleus for further rounds of transport. Back in the nucleus, Ran–GDP is converted to Ran–GTP by the guanine nucleotide exchange factor RCC1.[5]

Fig-8 The nucleocytoplasmic shuttling cycle of importin α.png

Fig-8:The nucleocytoplasmic shuttling cycle of importin α. (i) Importin α (α) forms a ternary complex with importin β (β) and cargo (blue circles). (ii) The ternary complex docks at the nuclear-pore complex (NPC) and (iii)translocates into the nucleus. (iv) Binding of Ran–GTP triggers the dissociation of the ternary complex. (v)Importin α binds to the exportin CAS–Ran–GTP complex and is exported to the cytoplasm. (vi) Ran–GAP-stimulated hydrolysis of GTP by Ran triggers the dissociation of the exportin complex and releases free importin α into the cytoplasm for another transport cycle. The recycling of importin β to the cytoplasm, and of Ran–GDP to the nucleus and its conversion to Ran–GTP are not shown.

Labs working on this gene

  • National Institute of Agrobiological Resources, Tsukuba, Ibaraki 305-8602, Japan.
  • Friedrich Miescher Institute, Maulbeerstrasse 66, 4058 Basel, Switzerland.
  • Central Research Institute of Electric Power Industry, Chiba, Japan.
  • School of Chemistry and Molecular Biosciences and Institute for Molecular Bioscience, University of Queensland, Brisbane Qld 4072, Australia.
  • School of Chemistry and Molecular Biosciences, Institute for Molecular Bioscience, and Australian Infectious Diseases Research Centre, University of Queensland, Brisbane, Qld 4072, Australia.
  • The State Key Laboratory of Protein and Plant Gene Research, College of Life Sciences, Peking University, Beijing, China.
  • Department of Biochemistry and Graduate Institute of Life Sciences, National Defense Medical Center, Taipei, Taiwan 114, Republic of China.


  1. 1.0 1.1 Jiang CJ, Imamoto N, Matsuki R, Yoneda Y, Yamamoto N. Functional characterization of a plant importin alpha homologue. J Biol Chem. 1998 Sep 11.
  2. 2.0 2.1 Jiang CJ, Shoji K, Matsuki R, Baba A, Inagaki N, Ban H, Iwasaki T, Imamoto N, Yoneda Y, Deng XW, Yamamoto N. Molecular cloning of a novel importin alpha homologue from rice, by which constitutive photomorphogenic 1 (COP1) nuclear localization signal (NLS)-protein is preferentially nuclear imported. J Biol Chem. 2001 Mar 23;276(12):9322-9. Epub 2000 Dec 20.
  3. 3.0 3.1 3.2 Guerra-Peraza O, Kirk D, Seltzer V, Veluthambi K, Schmit AC, Hohn T, Herzog E. Coat proteins of Rice tungro bacilliform virus and Mungbean yellow mosaic virus contain multiple nuclear-localization signals and interact with importin alpha. J Gen Virol. 2005 Jun;86(Pt 6):1815-26.
  4. 4.0 4.1 Shoji K, Iwasaki T, Matsuki R, Miyao M, Yamamoto N. Cloning of a cDNA encoding an importin-alpha and down-regulation of the gene by light in rice leaves. Gene. 1998 Jun 8;212(2):279-86.
  5. 5.0 5.1 5.2 5.3 5.4 5.5 Chang CW1, Couñago RL, Williams SJ, Bodén M, Kobe B.Crystal structure of rice importin-α and structural basis of its interaction with plant-specific nuclear localization signals.Plant Cell. 2012 Dec;24(12):5074-88. doi: 10.1105/tpc.112.104422. Epub 2012 Dec 18.
  6. 6.0 6.1 Iwasaki T, Matsuki R, Shoji K, Sanmiya K, Miyao M, Yamamoto N. A novel importin alpha from rice, a component involved in the process of nuclear protein transport. FEBS Lett. 1998 May 29;428(3):259-62.
  7. Goldfarb DS, Corbett AH, Mason DA, Harreman MT, Adam SA. Importin alpha: a multipurpose nuclear-transport receptor.Trends Cell Biol. 2004 Sep;14(9):505-14.

Structured Information