Homologous chromosomes are stably conjoined for Drosophila male meiosis I by SUM, a multimerized protein assembly with modules for DNA-binding and for separase-mediated dissociation co-opted from cohesin

Abstract

For meiosis I, homologous chromosomes must be paired into bivalents. Maintenance of homolog conjunction in bivalents until anaphase I depends on crossovers in canonical meiosis. However, instead of crossovers, an alternative system achieves homolog conjunction during the achiasmate male meiosis of Drosophila melanogaster. The proteins SNM, UNO and MNM are likely constituents of a physical linkage that conjoins homologs in D. melanogaster spermatocytes. Here, we report that SNM binds tightly to the C-terminal region of UNO. This interaction is homologous to that of the cohesin subunits stromalin/Scc3/STAG and α-kleisin, as revealed by sequence similarities, structure modeling and cross-link mass spectrometry. Importantly, purified SU_C, the heterodimeric complex of SNM and the C-terminal region of UNO, displayed DNA-binding in vitro. DNA-binding was severely impaired by mutational elimination of positively charged residues from the C-terminal helix of UNO. Phenotypic analyses in flies fully confirmed the physiological relevance of this basic helix for chromosome-binding and homolog conjunction during male meiosis. Beyond DNA, SU_C also bound MNM, one of many isoforms expressed from the complex mod(mdg4) locus. This binding of MNM to SU_C was mediated by the MNM-specific C-terminal region, while the purified N-terminal part common to all Mod(mdg4) isoforms multimerized into hexamers in vitro. Similarly, the UNO N-terminal domain formed tetramers in vitro. Thus, we suggest that multimerization confers to SUM, the assemblies composed of SNM, UNO and MNM, the capacity to conjoin homologous chromosomes stably by the resultant multivalent DNA-binding. Moreover, to permit homolog separation during anaphase I, SUM is dissociated by separase, since UNO, the α-kleisin-related protein, includes a separase cleavage site. In support of this proposal, we demonstrate that UNO cleavage by tobacco etch virus protease is sufficient to release homolog conjunction in vivo after mutational exchange of the separase cleavage site with that of the bio-orthogonal protease.

Nuclear elongation during spermiogenesis depends on physical linkage of nuclear pore complexes to bundled microtubules by Drosophila Mst27D

“…Therefore, alternative expression of Nslmb-vhh4-GFP using the transgenes betaTub85DP-Nslmb-vhh4-GFP and exumP-Nslmb-vhh4-GFP transgenes was explored ( S6 Fig ). These transgenes exploit spermatocyte-specific cis -regulatory sequences derived from the gene loci betaTub85D and exuperantia (exu) , respectively [ 53 , 54 ]. The transgene under control of exumP ( exumP-deGrad in the following) resulted in earlier degradation during spermatocyte maturation and worked better for EGFP-Nup358 elimination just before the onset of g-Mst27D-mCherry expression ( S5 Fig ).…”

A molecular cell biology toolkit for the study of meiosis in the silkworm Bombyx mori

“…In both Drosophila males and Lepidoptera females, the heterogametic sex utilizes protein aggregates to ensure accurate segregation of their chromosomes during meiosis. In the case of Drosophila males, proteins are used to create pseudo-chiasmata on each chromosome pair ( Cooper 1964 ; Kabakci, Reichle, et al 2022 ; Kabakci, Yamada, et al . 2022 ; Vernizzi and Lehner 2022 ).…”

Uncharted territories: Solving the mysteries of male meiosis in flies

“…After CT formation, homolog-homolog interactions are stabilized by the AHC pathway [8][9][10]. These recent studies demonstrate that AHC SUM proteins are Cohesin-related proteins that multimerize, bind directly to DNA in a sequenceindependent manner, and are cleaved by Separase for homolog separation [42,43]. Importantly, SUM proteins clearly link homologs in a mechanism similar to canonical COs.…”

“…In 2 recent studies, Kabakci and colleagues investigated how AHC proteins assemble and bind to DNA [42,43]. These studies had several key findings.…”

“…UNO is an α-kleisin-like protein analogous to Rad21 with a maintained Separase cleavage site [10]. Interestingly, the conjunction made by SUM proteins does not seem to involve DNA entrapment, as it does with canonical Cohesin, but rather direct DNA binding of SUM multimers in a sequence-nonspecific manner [42].…”