The histones H2A/H2B and H3/H4 are imported into the yeast nucleus by different mechanisms.

Proteins are imported from the cytoplasm into the nucleus by importin beta-related transport receptors. The yeast Saccharomyces cerevisiae contains ten of these importins, but only two of them are essential. After transfer through the nuclear pore, importins release their cargo upon binding to the Ran GTPase, the key regulator of nuclear transport. We investigated the import of the core histones in yeast and found that four importins are involved. The essential Pse1p and the nonessential importins Kap114p, Kap104p, and Yrb4p/Kap123p specifically bind to histones H2A and H2B. Release of H2 histones from importins requires Ran-GTP and DNA simultaneously suggesting a function of the importins in intranuclear targeting. H3 and H4 associate mainly with Pse1p and the dissociation requires Ran but not DNA, which points to a different import mechanism. Import of green fluorescent protein fusions to H2A and H2B requires primarily Pse1p and Kap114p, whereas Yrb4p plays an auxiliary role. Pse1p is predominantly necessary for nuclear uptake of H3 and H4, while Kap104p and Yrb4p also support import. We conclude from our in vivo and in vitro experiments that import of the essential histones is mediated mainly by the essential importin Pse1p, while the non-essential Kap114p functions in a parallel import pathway for H2A and H2B.