Evidence for the nuclear import of histones H3.1 and H4 as monomers

Abstract

Newly synthesised histones are thought to dimerise in the cytosol and undergo nuclear import in complex with histone chaperones. Here, we provide evidence that human H3.1 and H4 are imported into the nucleus as monomers. Using a tether‐and‐release system to study the import dynamics of newly synthesised histones, we find that cytosolic H3.1 and H4 can be maintained as stable monomeric units. Cytosolically tethered histones are bound to importin‐alpha proteins (predominantly IPO4), but not to histone‐specific chaperones NASP, ASF1a, RbAp46 (RBBP7) or HAT1, which reside in the nucleus in interphase cells. Release of monomeric histones from their cytosolic tether results in rapid nuclear translocation, IPO4 dissociation and incorporation into chromatin at sites of replication. Quantitative analysis of histones bound to individual chaperones reveals an excess of H3 specifically associated with sNASP, suggesting that NASP maintains a soluble, monomeric pool of H3 within the nucleus and may act as a nuclear receptor for newly imported histone. In summary, we propose that histones H3 and H4 are rapidly imported as monomeric units, forming heterodimers in the nucleus rather than the cytosol.

Chasing Tails: Cathepsin-L Improves Structural Analysis of Histones by HX-MS*[S]

“…However, the EX1 kinetics detected in our study for monomeric histones are extensive and expand the entire protein sequence (omitting the N-terminal tails), suggesting high conformational flexibility, in contrast to histones engaged in complexes where EX1 behavior is confined to short regions. Considering recent evidence showing that H3 and H4 monomers associate with importin-beta proteins and are imported into the nucleus in a monomeric state expands their interaction network further (70). We presume that a high degree of flexibility is a feature of histones required to counterbalance the breadth of chaperones that regulate their nuclear import and assembly into nucleosomes and provide exciting directions for future studies.…”

“…Most histone chaperones have a preference for binding and importing histone heterodimers (69), and HX structural information has been focused mainly on the analysis of heterodimers (34), tetramers (29) and nucleosomes (30,31) or on heterodimer-chaperone interactions (32)(33)(34)(35)(36), disregarding monomeric states. Recent evidence, however, has shown that H3 and H4 are predominantly monomeric in the cytosol and can be rapidly imported into the nucleus bound tightly to importin-␤ proteins (70). Obtaining structural information at the monomer level, therefore, is crucial for delineating structural elements governing histone-chaperone interactions.…”

Origin of the nuclear proteome on the basis of pre-existing nuclear localization signals in prokaryotic proteins

“…Such NLSs, even if they are present in prokaryotic proteins, can interact with karyopherins. Karyopherins have many functions in the cell and, in particular, can act as chaperones [26,27]. The protein domains interacting with karyopherins might have evolved before the origin of the nuclear envelope, with these domains containing sequences that potentially play a role in NLSs.…”

Origin of the nuclear proteome on the basis of pre-existing nuclear localization signals in prokaryotic proteins

“…Karyopherins have many functions in the cell and, in particular, can act as chaperones [26,27]. The protein domains interacting with karyopherins might have evolved before the origin of the nuclear envelope, and these domains contained sequences that can potentially play a role in NLSs.…”