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A more recent version of this article appeared on September 1, 2002
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Submitted on February 5, 2002
Revised on May 15, 2002
Accepted on June 13, 2002
1 Department of Cell and Molecular Biology, Feinberg School of Medicine, Northwestern University (present address: Department of Molecular and Cell Biology, University of California, Berkeley, 401 Barker Hall, #3204, Berkeley, CA 94720-3204)
2 Department of Cell and Molecular Biology, Feinberg School of Medicine, Northwestern University
3 Department of Cell and Molecular Biology, Feinberg School of Medicine, Northwestern University (present address: Akceli, Inc., One Hampshire St., 5th floor, Cambridge, MA 02139)
* Corresponding author. E-mail address: s-adam{at}northwestern.edu.
The importin 
family of nuclear-cytoplasmic transport factors mediates the nuclear localization of proteins containing classical nuclear localization signals. Metazoan animals express multiple importin proteins suggesting their possible roles in cell differentiation and development. Adult Caenorhabditis elegans hermaphrodites express 3 importin proteins, IMA-1, IMA-2 and IMA-3, each with a distinct expression and localization pattern. IMA-2 was expressed exclusively in germ line cells from the early embryonic through adult stages. The protein has a dynamic pattern of localization dependent on the stage of the cell cycle. In interphase germ cells and embryonic cells, IMA-2 is cytoplasmic and nuclear envelope- associated whereas in developing oocytes, the protein is cytoplasmic and intranuclear. During mitosis in germ line cells and embryos, IMA-2 surrounded the condensed chromosomes, but was not directly associated with the mitotic spindle. The timing of IMA-2 nuclear localization suggested that the protein surrounded the chromosomes after fenestration of the nuclear envelope in prometaphase. Depletion of IMA-2 by RNAi resulted in embryonic lethality and a terminal aneuploid phenotype. ima-2(RNAi) embryos have severe defects in nuclear envelope formation accumulating nucleoporins and lamin in the cytoplasm. We conclude that IMA-2 is required for proper chromosome dynamics in germ line and early embryonic mitosis and is involved in nuclear envelope assembly at the conclusion of mitosis.
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