The molecular basis of telomere-mediated chromosome pairing and genetic exchange during prophase I of meiosis

Abstract

Prophase I encapsulates the unique and defining events of meiosis; chromosome pairing, homologous recombination, synapsis, and subsequent segregation, to produce genetically unique haploid germ cells. These essential processes depend on a variety of protein complexes including the meiotic telomere complex (MTC), the synaptonemal complex (SC) and the meiotic recombination machinery. To carry out homology searches chromosomes must be tethered to the nuclear envelope, this is achieved by interplay between the MTC proteins (MAJIN, TERB1, and TERB2) and the shelterin complex protein, TRF1. Once attached, chromosomes undergo rapid prophase movements to find their homologous partner and begin to synapse. This involves the formation of the universally conserved SC structure along the lengths of aligned homologues. The SC provides the essential structural framework for HR and the crossover (CO) pathway. CO formation is dependent on the pro-CO machinery, including the E3 ligase proteins, HEI10 and RNF212. The main focus of this thesis is to use biophysical and structural approaches to deepen our understanding of the roles and mechanisms of the proteins and protein complexes involved in these meiotic processes. Here, we report the structural basis of the mammalian MTC and provide a mechanistic insight into chromosome tethering at the inner nuclear membrane (INM) achieved by the MTC. We show that the MTC recruits telomere-bound TRF1, through the 2:1 TRF1:TERB1 interaction, and undergoes subsequent structural rearrangement to displace TRF1 allowing the MTC to directly bind telomeric DNA and subsequently stabilise telomere-INM connectivity. The core architecture of mammalian SC is provided through the self-assembly of the transverse filament (TF) protein, SYCP1. We provide the first structural analysis of the D. melanogaster SC, specifically the TF protein, C(3)G. Biophysical analysis reveals that the central α-helical domain of C(3)G form dimers in a side-by-side parallel arrangement, but has some propensity to tetramerise, which could serve as building blocks for the recruitment and assembly of the complete SC. We show that HEI10 forms an obligate tetrameric structure and RNF212:RNF212b for a highly stable 2:2 complex and propose a structural model for the human E3 ligase proteins based upon solution scattering studies. Together, these findings provide a solid foundation for elucidating the mechanisms of mammalian meiosis.