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Nature Structural & Molecular Biology 13, 500–508 (1 June 2006) | doi:10.1038/nsmb1098

Importin-|[alpha]|-16 is a translocon-associated protein involved in sorting membrane proteins to the nuclear envelope

Suraj Saksena , Max D Summers , Jared K Burks , Arthur E Johnson & Sharon C Braunagel

A viral inner nuclear membrane–sorting motif sequence (INM-SM) was used to identify proteins that recognize integral membrane proteins destined for the INM. Herein we describe importin-α-16, a membrane-associated isoform of Spodoptera frugiperda importin-α that contains the C-terminal amino acid residues comprising armadillo helical-repeat domains 7–10. In the endoplasmic reticulum (ER) membrane, importin-α-16 is adjacent to the translocon protein Sec61α. Importin-α-16 cross-links to the INM-SM sequence as it emerges from the ribosomal tunnel and remains adjacent to the INM-SM after INM-SM integration into the ER membrane and release from the translocon. Cross-linking results suggest that importin-α-16 discriminates between INM- and non–INM-directed proteins. Thus, it seems that during and after cotranslational membrane integration, importin-α-16 is involved in the trafficking of integral membrane proteins to the INM.