transportin

Transportin is a family of nuclear transport receptors that belong to the karyopherin beta superfamily. These proteins mediate the import of specific cargoes from the cytoplasm into the nucleus by ferrying them through the nuclear pore complex in a process driven by the Ran GTPase gradient. Transportin receptors recognize non-classical nuclear localization signals on their cargoes, allowing selective and regulated nuclear import.

In humans, the best characterized members are Transportin-1 (TNPO1), Transportin-2 (TNPO2), and Transportin-3 (TNPO3). These proteins are composed of HEAT repeats that enable flexible cargo binding and transport. Cargo recognition relies on distinct signal motifs, including PY-rich nuclear localization signals (PY-NLS) and M9-type signals found in certain RNA-binding proteins. The different transportins have overlapping but distinct cargo repertoires, contributing to the selective localization of diverse nuclear proteins.

Cargoes vary by transportin member but commonly include RNA-binding proteins and splicing factors that participate in RNA processing and gene expression. For example, TNPO1 is known to import proteins with PY-NLS or M9-type signals, whereas TNPO2 and TNPO3 have additional cargo specificities that reflect their roles in cellular RNA metabolism and nuclear functions. The precise cargo set can be cell-type and context-dependent.

Biological significance of transportins lies in maintaining proper nucleocytoplasmic partitioning, which is essential for gene expression regulation and RNA processing. Disruption of transportin function or expression can perturb nuclear localization of key proteins and has been linked to cellular dysfunction and disease states. Transportins are conserved across eukaryotes and complement other import receptors in delivering proteins to the nucleus.