IPO13

Importin-13 is a protein encoded by the IPO13 gene in humans. Importin-13 is a member of the importin-β family of nuclear transport receptors (NTRs) and was first identified as a transport receptor in 2000. According to PSI-blast based secondary structure PREDiction (PSIPRED), importin-13 contains 38 α-helices. Importin-13 accommodates a range of cargoes due to its flexible superhelical structure and a cargo binding and release system that is distinct from other importin-like transport receptors. IPO13 is broadly expressed in a variety of tissues in the human body, including the heart, cornea, fetal lung, brain, endometrial carcinoma, and testes.

== Nucleocytoplasmic transport == In eukaryotic cells, macromolecules larger than ~40 kDa are actively transported between the nuclear and cytosolic compartment through nuclear pore complexes (NPCs) via soluble transport receptors. Importin-β-like factors are the largest class of NTRs and are classified as importins or exportins based on the direction of cargo transport and their response to the RanGTP gradient across the nuclear envelope. Importins load their cargoes in the cytoplasm in the absence of RanGTP and release their cargoes in the nucleus upon binding RanGTP. Exportins bind their cargoes in the nucleus in the presence of RanGTP and release their cargoes in the cytoplasm upon RanGTP hydrolysis. Unlike other importins, importin-13 is a versatile nuclear transport receptor with a bidirectional transport capacity, such that it mediates the nuclear import of certain cargoes and the nuclear export of other cargoes. Other nuclear transport receptors reported to facilitate both the nuclear import and export of macromolecules are exportin 5 (Msn5 in yeast) and exportin 4.