A stand-alone ICln release module revealed by deconstructing the SMN complex with engineered Brr1

The assembly of the Sm core, a ring-shaped protein complex essential for spliceosomal small nuclear ribonucleoprotein particle biogenesis, is chaperoned by the SMN complex in most eukaryotes. Intriguingly, the budding yeast Saccharomyces cerevisiae possesses a simplified version of this machinery-a single protein, Brr1-raising the question of whether it can functionally replace the entire multi-subunit SMN complex. Here, we engineered Brr1 (eBrr1) to bind Schizosaccharomyces pombe Sm proteins D1/D2/F/E/G (5Sm) and introduced it into fission yeast, which relies on a five-membered SMN complex. We demonstrate that eBrr1 functionally substitutes for the essential Gemin2 subunit. Surprisingly, in this context, the Gemin2-binding domain of SMN becomes dispensable, revealing that a stable SMN-Gemin2 interaction is not required for viability. Furthermore, we show that the SMN/Gemin6-8 subcomplex alone is sufficient to displace the assembly chaperone ICln from the ICln/5Sm/eBrr1 complex. Our findings provide key mechanistic insights into the stepwise action of the SMN complex and propose an evolutionary model in which the ICln-release machinery predated the stable incorporation of Gemin2.