Ectopic expression of meiotic cohesin RAD21L promotes adjacency of homologous chromosomes in somatic cells

Pairing, synapsis, and crossover recombination of homologous chromosomes (homologs) are prerequisite for the proper segregation of homologs during meiosis I. The meiosis-specific cohesin subunit, RAD21L, is essential for such meiotic chromosomal events, but it is unknown to what extent RAD21L by itself contributes to the process since various meiotic genes are also involved. To reveal the exclusive contribution of RAD21L to the specific regulation of homologs, we examined the effects of ectopic RAD21L expression on chromosome dynamics in somatic cells. We found that expression of GFP-fused RAD21L by plasmid transfection significantly shortened the distance between two FISH signals representing a pair of homologs for chromosome X or chromosome 11 in the nuclei compared to that in control (non-transfected) cells whereas expression of GFP-fused RAD21, a mitotic counterpart of RAD21L, showed no detectable effects. This indicates that RAD21L, when ectopically expressed in somatic cells, can promote homolog adjacency, which resembles the homolog pairing normally seen during meiosis. Furthermore, deletion of the N-terminal winged helix domain from RAD21L, prevented its association with another cohesin subunit, SMC3, and abolished the phenomenon of homolog adjacency upon ectopic expression. Our findings suggest that RAD21L-containing cohesin can promote homolog adjacency independently of other meiotic gene products, which might be central to the process of meiotic homolog paring.