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

Source: PubMed Central Open Access, NCBI / U.S. National Library of Medicine

Abstract 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 yeastpossesses 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 bindSm 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. Graphical Abstract Graphical Abstract For image description, please refer to the figure legend and surrounding text. http://www.w3.org/1999/xlink float portrait gkag524figgra1.jpg float ga1 portrait graphical