Molecular basis for import cargo discrimination by importin α isoforms (793.2)

The human genome encodes seven isoforms of importin α grouped into three subfamilies, α1, α2 and α3. All isoforms have an N‐terminal auto‐inhibitory Importin β binding (IBB)‐domain and a C‐terminal helical core that associates with cargos bearing a Nuclear Localization Signal (NLS‐cargos). Although all importin α isoforms can associate with classical NLS‐cargos, certain cargos are selectively imported by specific isoforms. This specificity is not seen in vitro, as NLS‐cargos bind different ΔIBB isoforms with comparable affinity. To identify rules for importin α selection, we have determined high‐resolution crystal structures of representative isoforms from each family of importin α in complex with a model NLS‐cargo. In parallel, structure‐based mutagenesis and an in vitro binding assay were used to examine the degree of IBB‐dependent auto‐inhibition among different isoforms. This work led us to three findings: (i) all isoforms share an identical NLS‐binding groove across the inner surface of ARM 2‐8. (ii) Although the NLS‐binding groove is invariant, importin α isoforms present different curvature, which positions NLS‐cargos with subtly different binding energetics. (iii) Importin α isoforms are differentially auto‐inhibited by their IBB‐domains. We propose that these three factors determine NLS‐cargo discrimination and explain the specificity for importin α isoforms observed in living cells. Grant Funding Source : Supported by T32 GM 100836‐2