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Karyopherin-mediated import of integral inner nuclear membrane proteins

Abstract

Targeting of newly synthesized integral membrane proteins to the appropriate cellular compartment is specified by discrete sequence elements, many of which have been well characterized. An understanding of the signals required to direct integral membrane proteins to the inner nuclear membrane (INM) remains a notable exception. Here we show that integral INM proteins possess basic sequence motifs that resemble ‘classical’ nuclear localization signals. These sequences can mediate direct binding to karyopherin-α and are essential for the passage of integral membrane proteins to the INM. Furthermore, karyopherin-α, karyopherin-β1 and the Ran GTPase cycle are required for INM targeting, underscoring parallels between mechanisms governing the targeting of integral INM proteins and soluble nuclear transport. We also provide evidence that specific nuclear pore complex proteins contribute to this process, suggesting a role for signal-mediated alterations in the nuclear pore complex to allow for passage of INM proteins along the pore membrane.

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Figure 1: Heh1–YFP and Heh2–YFP localize to the INM in a Ran-dependent manner.
Figure 2: Kap60 directly interacts with Heh2 and is required for nuclear envelope targeting.
Figure 3: Kap60 NLSs are required for the INM localization of Heh2.
Figure 4: Heh1–YFP and Heh2–YFP are mistargeted in the absence of Nup2 and Nup170.

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Acknowledgements

We are grateful to S. Wente, R. Wozniak, X. Zhao, M. Winey and K. Belanger for yeast strains and plasmids, and M. Rout and J. Novatt for the anti-GFP antibody and Kap95. We also thank A. North for help with the spinning disk confocal microscope, R. Peters and L. Gerace for discussions, and E. Wren for helping us launch this project. We are especially indebted to H. Shio for electron micrograph technical support. This work was supported by an NIH fellowship (to M.C.K.) and the Howard Hughes Medical Institute (to G.B. and C.P.L.).

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Correspondence to Megan C. King or C. Lusk.

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King, M., Lusk, C. & Blobel, G. Karyopherin-mediated import of integral inner nuclear membrane proteins. Nature 442, 1003–1007 (2006). https://doi.org/10.1038/nature05075

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