Cell Biology

The Ins and Outs of Import

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Science  25 Jun 2010:
Vol. 328, Issue 5986, pp. 1613
DOI: 10.1126/science.328.5986.1613-d
CREDIT: DISHINGER ET AL., NAT. CELL BIOL. 12, 10.1038/NCB2073 (2010)

There are several different mechanisms available for moving proteins to where they need to be. In eukaryotes, nuclear proteins, which are synthesized in the cytoplasm, are transported into the nucleus by means of nuclear localization signals encoded in their amino acid sequence; these signals are recognized by receptors of the importin/karyopherin family, which directionally ferry their protein cargo from the cytoplasm into the nucleus with the aid of the small GTPase Ran. A gradient of accessory proteins across the nuclear membrane produces a corresponding gradient of the GTP- and GDP-bound forms of Ran. In the cytoplasm, where there are low levels of RanGTP, the import receptors bind their cargoes and carry them across the nuclear envelope. Upon reaching the nucleus, the receptors encounter high levels of RanGTP and release their loads.

Dishinger et al. find that this mode of conveyance is also used to transport molecules to a distant part of the cell. They identified a sequence within a kinesin motor protein, KIF17 (green), that targets it to the primary cilium (red), which is an organelle protruding from the cell surface. A RanGTP/GDP gradient between the cilia and the cytoplasm was required for the localization of KIF17 to the cilia. Furthermore, a member of the importin family of receptors bound KIF17, and this interaction was sensitive to RanGTP in vitro, thus revealing shared laborers in nuclear and ciliary import.

Nat. Cell Biol. 12, 10.1038/ncb2073 (2010).

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