Effect of nuclear import receptors on liquid-liquid phase separation

Low-complexity (LC) sequences, regions that are predominantly made up of limited amino acids, are often observed in eukaryotic nuclear proteins. The role of these LC sequences has remained unclear for decades. Recent studies have shown that LC sequences are important in the formation of membrane-less organelles via liquid-liquid phase separation (LLPS). The RNA binding protein, fused in sarcoma (FUS), is the most widely studied of the proteins that undergo LLPS. It forms droplets, fibers, or hydrogels using its LC sequences. The N-terminal LC sequence of FUS is made up of Ser, Tyr, Gly, and Gln, which form a labile cross-beta polymer core while the C-terminal Arg-Gly-Gly repeats accelerate LLPS. Normally, FUS localizes to the nucleus via the nuclear import receptor karyopherin beta 2 (Kap beta 2) with the help of its C-terminal proline-tyrosine nuclear localization signal (PY-NLS). Recent findings revealed that Kap beta 2 blocks FUS mediated LLPS, suggesting that Kap beta 2 is not only a transport protein but also a chaperone which regulates LLPS during the formation of membrane-less organelles. In this review, we discuss the effects of the nuclear import receptors on LLPS.