Nuclear Slalom: Importin Run

A nod to Horace Goes Skiing (William Tang, Beam Software, 1983), made in Melbourne, where Horace crossed a busy road to hire his skis and then took on a slalom course. Here the road is the cytoplasm, two screens wide, and the traffic is the cell’s own: motor proteins hauling vesicles along their tracks, ribosomes, mitochondria, proteasomes and, later on, viruses. The ski run goes down through a nuclear pore into the nucleus, and the cable car brings you back up. There are four orders, each with a different cargo and a different way in, and what each cargo does changes the rest of the game.

The science

• You are importin β, a courier that carries proteins into the nucleus. It’s a long, curling chain of about 19 HEAT repeats, each a pair of helices, which is how the game draws it.
• Importin α is the adaptor. Many cargoes carry a short “postcode”, a nuclear localisation signal (NLS). The first one found was PKKKRKV, in the SV40 large T antigen. Importin α reads it with a crescent of ten armadillo (ARM) repeats, three helices each; the NLS lies along the groove inside the curve (the gold helices in the game show where each cargo binds). Its other end, the IBB domain, reaches up into importin β.
• Different importin αs for different jobs. Humans have several importin αs, with famously muddled names (importin α1 is made by the KPNA2 gene, α3 by KPNA4, α5 by KPNA1). Some cargoes prefer one of them, so in the game you choose:
  • a ribosomal protein needs no adaptor: importin β grabs it directly (Jäkel and Görlich, 1998). Ribosomes, with about 80 proteins each, are put together in the nucleolus. In the game, more ribosomes mean a new importin β: an extra life;
  • SOX2, a pioneer factor that can open up tightly packed chromatin and keeps neural stem cells going, has two NLSs that together span nine of importin α3’s ten ARM repeats, which explains why it prefers α3 (Jagga et al., 2021). In the game it opens the chromatin, and the ski run gets clearer;
  • the Hendra virus W protein binds importin α3 through a larger surface and over 50 times more tightly than importin α1 (Smith et al., 2018; see also the structure page). Once in the nucleus, W blocks the signals that switch on interferon, the cell’s antiviral alarm (Shaw et al., 2005). In the game the alarm goes off and the viruses multiply;
  • STAT1 is switched on when interferon from neighbouring cells reaches the cell’s receptor and JAK kinases add phosphates. Two phosphorylated STAT1s pair up and are carried in by importin α5, not by importin α1 (Sekimoto et al., 1997), through an unusual signal near the end of the ARM repeats. Inside, they switch on interferon-stimulated genes. Henipavirus W proteins also bind STAT1 and hold on to it in the nucleus (Shaw et al., 2004): those are the red spiky things on the last ski run.
• The pore. A nuclear pore complex is a ring of about 1,000 protein molecules from about 30 kinds of nucleoporin, weighing about 120 MDa, set in the double membrane of the nuclear envelope. Its channel is filled with floppy FG repeats (phenylalanine–glycine) that keep big molecules out unless an importin carries them: the importin binds the FG repeats as it slips through (your FG flags). Each pore can handle up to 1,000 crossings a second.
• What makes it one-way: Ran. Inside the nucleus, RCC1, sitting on chromatin, loads the small protein Ran with GTP. RanGTP grabs importin β just inside the pore’s basket and makes it let go of the cargo. Importin β rides back out with RanGTP, and importin α with its own exporter, CAS (exportin-2). On the cytoplasm side, RanGAP, held on the pore’s filaments by RanBP2, makes Ran use up its GTP, freeing the importins for another trip, and NTF2 takes RanGDP back in. That’s the cable car.
• More importin stories from our papers with Jade Forwood’s group at Charles Sturt University: MERS coronavirus ORF4b binds importin α in an unusual way, right where NF-κB, another alarm signal, would bind (Munasinghe et al., 2022); adding a phosphate next to the NLS of a cytomegalovirus protein, UL44, makes it bind importin α ten times less tightly (Cross et al., 2024); and the capsid of beak and feather disease virus has a nuclear-targeting arm (structure page).
• A Nobel Prize. Günter Blobel won the 1999 Nobel Prize in Physiology or Medicine for discovering that proteins carry signals that send them to the right place in the cell.

Where the game cuts corners

• Proteins don’t walk, ski or wear woolly hats. Everything in a cell gets about by diffusion, jostling at random, and crossing a pore takes a tiny fraction of a second, not a whole ski run.
• The cytoplasm really is crowded (macromolecular crowding), but motor proteins don’t run anyone over. The traffic rules are pure Horace.
• A cell doesn’t “choose” an importin α: each cargo simply binds best to the ones that fit its signal, and many cargoes can use several. The ribosomal proteins can also travel with other importins, such as transportin.
• RanGTP lets go of the cargo at the basket, and then the cargo makes its own way to the DNA. The game hands you its skis for that last bit.
• One SOX2 doesn’t open all the chromatin in the nucleus: pioneer factors open particular places. A clearer ski run is the game’s way of saying thank you.
• Hsp70 is a chaperone that helps proteins fold and refold, not an ambulance. Horace’s ambulance cost him money; here it costs a life.
• There’s no cable car. Importins go back out through the pores the same way they came in, and Ran’s gradient, GTP inside and GDP outside, keeps the traffic flowing in the right direction.