Signaling into the nucleus through the importin 7 pathway

Importin 7 (IPO7) is a nuclear transport receptor of the β-karyopherin family that mediates the translocation of a broad spectrum of macromolecules, commonly referred to as cargoes. Discovered nearly three decades ago, IPO7 was initially identified as an import receptor for constitutive cellular cargoes, including histone H1 and ribosomal proteins, and was shown to function synergistically and partially redundantly with canonical receptors such as importin β1 and karyopherin β2. Over the past 15 years, however, accumulating evidence has established IPO7 as an important mediator of signal-dependent nuclear trafficking in response to extracellular stimuli, including cytokines, growth factors, and cellular stress. Thus, IPO7 has emerged as a versatile nuclear transport receptor that couples extracellular signaling to dynamic changes in nuclear composition and gene expression. Mechanistically, many IPO7 cargoes lack classical nuclear localization signals and instead contain noncanonical targeting motifs that directly engage IPO7. In several cases, phosphorylation-dependent conformational changes expose these motifs, promoting IPO7 binding and translocation through the nuclear pore complex. The expanding repertoire of IPO7 cargoes, including ERK, SMAD3, EGR1, GLI1, the glucocorticoid receptor, HIF-1α, YAP1, and RUNX2, highlights its prominent role at the interface of signaling and transcriptional control. Consistent with these functions, dysregulation of IPO7-mediated transport has been implicated in cancer, hypoxia, and other pathological states. Beyond cellular signaling, IPO7 also contributes to the nuclear trafficking of viral genomes and proteins. Here, we review the molecular mechanisms of IPO7-dependent nuclear import, emerging principles of cargo recognition, and pathways that regulate IPO7 activity during cellular signaling.