I attended a talk on Signals and Blockers in Nuclear-Cytoplasmic Transport by Yuh Min Chook, a professor at the University of Texas Southwestern Medical Center. She was studying how the protein Karyopherin (Kap) mediates the nuclear-cytoplasmic transport of macromolecules, specifically how core histones for DNA wrapping are imported into the nucleus. There are three types of Kap: Importins (into the nucleus), Exportins (out of the nucleus), and Biportins (going both ways). Ran^GTP, which is in then nucleus, and Ran^GDP, which is in the cytoplasm,  work asymmetrically to control the direction of transport. Ran^GTP is ultimately the protein that binds importin/exportin—importin with negative cooperativity, and exportin with positive cooperativity.

Histones are synthesized and folded in the cytoplasm, and then imported into the nucleus in multiple parts before being fully assembled. H3,H4, H2A, and H2B are some of the main protein subunits that fold together to create histones. Importin-4 (IMP4) imports H3 and H4, which fold together to create their own dimer, and Importin-9 (IMP9) imports H2A and H2B, which also fold into their own dimer. The lab examined import into the nucleus, and assembly into histones.

First, IMP4 makes contact with the H3 protein’s N-terminal tail and uses that to transport it from the cytoplasm to nucleus. H4 is brought along since it is bound to H3 via ASF1, which stabilizes their fold but does not make contact with IMP4. Together, ASF1 and IMP4 shield the DNA binding domain of the H3-H4 tetramerization sites. Ran^GTP, which is only at a high concentration in the nucleus, acts as the trigger to release H3-H4 from the IMP4. IMP-9 wraps around the H2A-H2B dime and transports it into the nucleus. In the process, the histone tail does not make contact with IMP9 at all. They are chaperoned by KAP114 in the process, and once in the nucleus, Ran^GTP binding alters the Kap-114 histone interaction and exposes a key DNA binding region of the H2A-H2B dimer and enables it to transfer to assembly. Finally, the histone chaperone Nap1 binds and forms the Nap1 – H2A-H2B – Kap114 – Ran^GTP that shelters the histone and stimulates effective transfer to the nucleosome. The mechanisms for H2A-H2B and H3-H4 to bind to one another are still being studied at the lab, but these steps already tell us more than we knew and teach us a lot about possible paths for gene therapies in the future, or ways to alter DNA binding and thus epigenetics.