Sperm chromatin structure and reproductive fitness are altered by substitution of a single amino acid in mouse protamine 1.

Conventional dogma presumes that protamine-mediated DNA compaction in sperm is achieved by electrostatic interactions between DNA and the arginine-rich core of protamines. Phylogenetic analysis reveals several non-arginine residues conserved within, but not across species. The significance of these residues and their post-translational modifications are poorly understood. Here, we investigated the role of K49, a rodent-specific lysine residue in protamine 1 (P1) that is acetylated early in spermiogenesis and retained in sperm. In sperm, alanine substitution (P1(K49A)) decreases sperm motility and male fertility-defects that are not rescued by arginine substitution (P1(K49R)). In zygotes, P1(K49A) leads to premature male pronuclear decompaction, altered DNA replication, and embryonic arrest. In vitro, P1(K49A) decreases protamine-DNA binding and alters DNA compaction and decompaction kinetics. Hence, a single amino acid substitution outside the P1 arginine core is sufficient to profoundly alter protein function and developmental outcomes, suggesting that protamine non-arginine residues are essential for reproductive fitness.

The KASH5 protein involved in meiotic chromosomal movements is a novel dynein activating adaptor.

Dynein harnesses ATP hydrolysis to move cargo on microtubules in multiple biological contexts. Dynein meets a unique challenge in meiosis by moving chromosomes tethered to the nuclear envelope to facilitate homolog pairing essential for gametogenesis. Though processive dynein motility requires binding to an activating adaptor, the identity of the activating adaptor required for dynein to move meiotic chromosomes is unknown. We show that the meiosis-specific nuclear-envelope protein KASH5 is a dynein activating adaptor: KASH5 directly binds dynein using a mechanism conserved among activating adaptors and converts dynein into a processive motor. We map the dynein-binding surface of KASH5, identifying mutations that abrogate dynein binding in vitro and disrupt recruitment of the dynein machinery to the nuclear envelope in cultured cells and mouse spermatocytes in vivo. Our study identifies KASH5 as the first transmembrane dynein activating adaptor and provides molecular insights into how it activates dynein during meiosis.

Structure of a meiosis-specific complex central to BRCA2 localization at recombination sites.

Meiotic cells invoke breast cancer susceptibility gene 2 (BRCA2) to repair programmed double-stranded DNA breaks and accomplish homologous recombination. The meiosis-specific protein MEILB2 facilitates BRCA2 recruitment to meiotic recombination sites. Here, we combine crystallography, biochemical analysis and a mouse meiosis model to reveal a robust architecture that ensures meiotic BRCA2 recruitment. The crystal structure of the MEILB2-BRCA2 complex reveals how two MEILB2 homodimers sandwich two chains of BRCA2 to afford a 4:2 architecture. The sandwich lacks close contact between the two MEILB2 dimers or the two BRCA2 chains. Instead, the two halves of each BRCA2 chain bridge two MEILB2 subunits from different homodimers to form the MEILB2-BRCA2-MEILB2 sandwich. Several identical residues from the two MEILB2 subunits are employed to engage the BRCA2 halves, justifying their strict conservation. Mutational analysis of the interface reveals a synergistic mechanism for MEILB2-BRCA2 recruitment during meiosis. Overall, these studies demonstrate how BRCA2 efficiently localizes in the cell to facilitate meiosis.