Drosophila Yemanuclein and HIRA Cooperate for De Novo Assembly of H3.3-Containing Nucleosomes in the Male Pronucleus

Guillermo A Orsi,Régis E Meyer,Ounissa Aït-Ahmed,Michèle Capri,Ahmed Algazeery,Henri Gruffat,Pierre Couble,Laure M Sapey-Triomphe,Benjamin Loppin,Béatrice Horard

doi:10.1371/journal.pgen.1003285

Abstract

The differentiation of post-meiotic spermatids in animals is characterized by a unique reorganization of their nuclear architecture and chromatin composition. In many species, the formation of sperm nuclei involves the massive replacement of nucleosomes with protamines, followed by a phase of extreme nuclear compaction. At fertilization, the reconstitution of a nucleosome-based paternal chromatin after the removal of protamines requires the deposition of maternally provided histones before the first round of DNA replication. This process exclusively uses the histone H3 variant H3.3 and constitutes a unique case of genome-wide replication-independent (RI) de novo chromatin assembly. We had previously shown that the histone H3.3 chaperone HIRA plays a central role for paternal chromatin assembly in Drosophila. Although several conserved HIRA-interacting proteins have been identified from yeast to human, their conservation in Drosophila, as well as their actual implication in this highly peculiar RI nucleosome assembly process, is an open question. Here, we show that Yemanuclein (YEM), the Drosophila member of the Hpc2/Ubinuclein family, is essential for histone deposition in the male pronucleus. yem loss of function alleles affect male pronucleus formation in a way remarkably similar to Hira mutants and abolish RI paternal chromatin assembly. In addition, we demonstrate that HIRA and YEM proteins interact and are mutually dependent for their targeting to the decondensing male pronucleus. Finally, we show that the alternative ATRX/XNP-dependent H3.3 deposition pathway is not involved in paternal chromatin assembly, thus underlining the specific implication of the HIRA/YEM complex for this essential step of zygote formation.

Highlights

Of octameric nucleosomes in eukaryotic chromatin is a stepwise process where deposition of a histone H3-H4 heterotetramer precedes incorporation of two H2A-H2B dimers [1]
In eggs laid by yem mutant females, the male pronucleus fails to assemble its nucleosomes, resulting in the loss of paternal chromosomes at the first zygotic division
We found that YEM and HIRA are mutually dependent to perform chromatin assembly at fertilization, demonstrating that they tightly cooperate in vivo

Summary

Introduction

Of octameric nucleosomes in eukaryotic chromatin is a stepwise process where deposition of a histone H3-H4 heterotetramer precedes incorporation of two H2A-H2B dimers [1]. H3.3 is enriched at transcribed gene bodies, it is established that this histone is deposited at various chromatin regions, such as regulatory elements, mammalian telomere repeats or satellite DNA blocks [5,6,7,14,15,16,17,18]. This surprising versatility of H3.3 could reflect its ability to be deposited in regions that are subjected to nucleosome depletion or rapid histone turnover [5,7,19]

Methods

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Discussion

Conclusion