Authors
Yang, Z., Fu, Y., Mikhova, M., Chen, D., Hebert, C., Ye, A. Y., Lin, X., Williams, T., Hu, H., Ba, Z., Dekker, J., Alt, F. W.
Abstract
The cohesin ring complex organizes and mediates functional activity of the genome through chromatin loop extrusion1,2. Cohesin comprises a core SMC1-SMC3-RAD21 ring associated with multiple regulatory proteins, including SA1 and SA2 paralogs that bind mutually-exclusively to a RAD21 site3,4. Functional studies of cohesin complexes in vivo often employ inactivation of particular components2,3. However, inactivation of SA1 or SA2 in mice is limited by mouse embryonic lethality5-7 and, for dual SA1/2-inactivation by cycling cell lethality8-10. For these reasons, it remains unknown whether the requirement for SA1/2 in loop extrusion in vitro11,12 applies in vivo1,2, and whether these factors have overlapping or distinct functions in regulating this process3. Here, we employ G0/G1-arrested v-Abl pro-B lines, which are viable in the absence of cohesin-complex factors13,14, to dissect SA1 and SA2 functions in V(D)J recombination, a strictly G0/G1-specific process that harnesses loop extrusion to effect distinct Igh and Ig{kappa} locus recombination mechanisms15,16. Robust long-range Igh or Ig{kappa} V(D)J recombination continued unabated in the absence of SA1 or SA2. However, inactivation of both abrogated most positioned chromatin loops genome-wide and also abolished long-range Igh V(D)J recombination with a phenotype similar to RAD21 (cohesin) inactivation13. Yet, robust Ig{kappa} V(D)J recombination continued upon dual SA1/2-inactivation, which along with Hi-C findings, demonstrates ongoing loop-extrusion in the absence of SA1/2. Correspondingly, replacement of weak Igh V(D)J recombination signal sequences (RSSs) that target V(D)J recombination activity, with strong Ig{kappa} counterparts15 restored long-range Igh V(D)J recombination in SA1/2 inactivated cells. We conclude that SA1/2 together maintain loop extrusion impediments and genome-wide architecture in G0/G1-arrested cells; and, in their absence, loop extrusion proceeds unimpeded. Remarkably, strong RSSs allow Ig{kappa} to function normally in the absence of internal positioned loop structure across the 3.1-mega-base Ig{kappa} locus. We discuss implications of our findings for long-range gene regulation in G0/G1-arrested cells more broadly.
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bioRxiv
The authors list and abstract were imported from bioRxiv on 12 Oct 2026.
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