The linker histone H1.2 is a novel component of the nucleolar organizer regions

Junjie Chen,Boon Heng Dennis Teo,Yitian Cai,Seng Yin Kelly Wee,Jinhua Lu

doi:10.1074/jbc.m117.810184

Junjie Chen, Boon Heng Dennis Teo + Show 3 more

Open Access

https://doi.org/10.1074/jbc.m117.810184

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Abstract

The nucleoli accumulate rRNA genes and are the sites of rRNA synthesis and rRNA assembly into ribosomes. During mitosis, nucleoli dissociate, but nucleolar remnants remain on the rRNA gene loci, forming distinct nucleolar organizer regions (NORs). Little is known about the composition and structure of NORs, but upstream binding factor (UBF) has been established as its master organizer. In this study, we sought to establish new proteins in NORs. Using UBF-Sepharose to isolate UBF-binding proteins, we identified histone H1.2 as a candidate partner but were puzzled by this observation, given that UBF is known to be located predominantly in nucleoli, whereas H1.2 distributed broadly among the chromatins in interphase nuclei. We then examined cells undergoing mitosis and saw that both H1.2 and UBF were recruited into NORs in this state, reconciling the results of our UBF pulldowns. Inhibiting rRNA synthesis in interphase nuclei also induced NOR-like structures containing both UBF and H1.2. When chromosomes were isolated and spread on coverslips, NORs appeared separated from the chromosomes containing both UBF and H1.2. After chromosomes were fragmented by homogenization, intact NORs remained visible. Results collectively suggest that NORs are independent structures and that the linker histone H1.2 is a novel component of this structure.

Highlights

The nucleoli accumulate rRNA genes and are the sites of rRNA synthesis and rRNA assembly into ribosomes
Based on the tripartite model, an interphase nucleolus is composed of one or more fibrillar centers (FCs) each being surrounded by a dense fibrillar component (DFC), and these FC/DFC units are embedded in a greater granular component (GC) (8 –10)
The H1.2 globular domain (GD) alone showed no binding to upstream binding factor (UBF). These results suggest that H1.2 binds to UBF through its C-terminal domain (CTD), which is more diversified than GD among the H1 variants [37]

Summary

Introduction

The nucleoli accumulate rRNA genes and are the sites of rRNA synthesis and rRNA assembly into ribosomes. Nucleoli dissociate, but nucleolar remnants remain on the rRNA gene loci, forming distinct nucleolar organizer regions (NORs). Nucleoli disintegrate but remnants remain on rDNA, forming the nucleolar organizer regions (NORs) [4, 5]. Based on the tripartite model, an interphase nucleolus is composed of one or more fibrillar centers (FCs) each being surrounded by a dense fibrillar component (DFC), and these FC/DFC units are embedded in a greater granular component (GC) (8 –10). These nucleoli are encircled by a dense layer of perinucleolar heterochromatins, but chromatins are otherwise scarce in the nucleolar lumen [11, 12].

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Discussion

Conclusion