Biology subjects

Milano, C. R.

Publications and source records attributed to Milano, C. R..

3 recordsLinked to original sources

bioRxiv2026

Two distinct modes of meiotic chromosome synapsis

The pairwise alignment of homologous chromosomes within the synaptonemal complex (SC) is important for meiotic crossover recombination and fertility. However, chromosomes do not need sequence homology to synapse, with meiotic recombination defects often leading to synapsis of non-homologous chromosome segments. Here we show that such heterologous synapsis reflects a distinctly regulated mode of meiotic chromosome synapsis that also happens during the early stages of wild-type yeast meiosis and occurs in parallel to the well-known synapsis initiation at crossover-designated sites. Heterologous synapsis initiates along chromosome arms after double-strand break resection and is accompanied by canonical markers of crossover repair, but does not need recombinase-dependent strand invasion. Instead, it requires the DNA-damage sensor kinase ATR/Mec1, which phosphorylates of a specific amino acid in Zip1, the major transverse filament protein of the SC. Phospho-mimetic mutants in ZIP1 rescue the synapsis defect of mec1 mutants and also partially restore gamete viability, indicating that this particular MEC1 function is important for the faithful completion of meiosis. Importantly, crossover repair quickly rectifies heterologous synapsis, allowing successful completion of meiosis even when most of the genome initially synapses independently of homology. Our data identify meiotic chromosome synapsis as a dynamic and reversible process that becomes optimized as result of recombination-dependent chromosome pairing.

genetics↗

bioRxiv2023

Chromatin binding by HORMAD proteins regulates meiotic recombination initiation

The meiotic chromosome axis coordinates chromosome organization and interhomolog recombination in meiotic prophase and is essential for fertility. In S. cerevisiae, the HORMAD protein Hop1 mediates enrichment of axis proteins at nucleosome-rich genomic islands through a central chromatin-binding region (CBR). Here, we use cryoelectron microscopy to show that the Hop1 CBR directly recognizes bent nucleosomal DNA through a composite interface in its PHD and winged helix-turn-helix domains. Targeted disruption of the Hop1 CBR-nucleosome interface causes loss of axis proteins from nucleosome-rich islands, reduces meiotic DNA double-strand breaks (DSBs), and leads to defects in chromosome synapsis. Synthetic effects with the disassemblase Pch2 suggest that nucleosome binding delays a conformational switch in Hop1 from a DSB-promoting, Pch2-inaccessible state to a DSB-inactive, Pch2-accessible state to regulate the extent of meiotic DSB formation. Phylogenetic analyses of meiotic HORMADs reveal an ancient origin of this domain, suggesting that these mechanisms are broadly conserved.

cell biology↗

bioRxiv2019

Mutation of the ATPase domain of MutS homolog-5 (MSH5) reveals a requirement for a functional MutSγ complex for all crossovers in mammalian meiosis

During meiosis, induction of DNA double strand breaks (DSB) leads to recombination between homologous chromosomes, resulting in crossovers (CO) and non-crossovers (NCO). Only 10% DSBs resolve as COs, mostly through a class I pathway dependent on MutS{gamma} (MSH4/ MSH5). Class II CO events represent a minor proportion of the total CO count and also arise from DSBs, but are not thought to involve MutS{gamma}. However, loading of MutS{gamma} occurs very early in prophase I at a frequency that far exceeds the final number of class I COs found in late prophase I. Moreover, loss of MutS{gamma} in mouse results in apoptosis before CO formation, preventing analysis of its CO function. We generated a mutation in the ATP binding domain of Msh5 (Msh5GA). While this mutation was not expected to affect MutS{gamma} complex formation, MutS{gamma} foci do not accumulate during prophase I. Nevertheless, while some spermatocytes from Msh5-/- animals progress into pachynema, most spermatocytes from Msh5GA/GA mice progress to late pachynema and beyond. Some spermatocytes from Msh5GA/GA mice complete prophase I entirely, allowing for the first time an assessment of MSH5 function in CO formation. At pachynema, Msh5GA/GA spermatocytes show persistent DSBs, incomplete homolog pairing, and fail to accumulate MutL{gamma} (MLH1/MLH3). Unexpectedly, Msh5GA/GA diakinesis-staged spermatocytes have no chiasmata at all from any CO pathway, indicating that a functional MutS{gamma} complex in early prophase I is a pre-requisite for all COs. ARTICLE SUMMARYMSH4/MSH5 are critical components of the class I crossover (CO) machinery, which is responsible for >90% of the COs that arise in mammalian meiosis. We generated a point mutation in the ATP binding motif of Msh5, and found that mutant spermatocytes lose all COs, not just those arising from the class I pathway.

genetics↗