Histone Variant H2A.L.2 Guides Transition Protein-Dependent Protamine Assembly in Male Germ Cells

Histone replacement by transition proteins (TPs) and protamines (Prms) constitutes an essential step for the successful production of functional male gametes, yet nothing is known on the underlying functional interplay between histones, TPs, and Prms. Here, by studying spermatogenesis in the absence of a spermatid-specific histone variant, H2A.L.2, we discover a fundamental mechanism involved in the transformation of nucleosomes into nucleoprotamines. H2A.L.2 is synthesized at the same time as TPs and enables their loading onto the nucleosomes. TPs do not displace histones but rather drive the recruitment and processing of Prms, which are themselves responsible for histone eviction. Altogether, the incorporation of H2A.L.2 initiates and orchestrates a series of successive transitional states that ultimately shift to the fully compacted genome of the mature spermatozoa. Hence, the current view of histone-to-nucleoprotamine transition should be revisited and include an additional step with H2A.L.2 assembly prior to the action of TPs and Prms. [Display omitted] •H2A.L.2 and transition proteins (TPs) are co-expressed in post-meiotic germ cells•Lack of H2A.L.2 leads to defective spermatozoa genome compaction and male infertility•H2A.L.2 assembly opens nucleosomes, releases flexible DNA ends, and allows TP loading•TP loading drives protamine processing and organized compaction of the sperm genome Histone-to-protamine transition is essential for procreation, yet its molecular basis has remained obscure. Barral et al. show that a histone variant, H2A.L.2, directs the transformation of nucleosomes by allowing the loading onto chromatin of non-histone transition proteins, which in turn control the final genome organization and compaction by protamines.