Structural Insights into Negative Cooperativity between Gemin2 and RNA in Sm-class snRNP Assembly

Abstract

Sm-class ribonucleoprotein particles (RNPs) are ring-shaped structures (Sm cores) formed by Sm hetero-heptamer around a segment of RNA, containing a nonameric oligoribonucleotide, PuAUUUNUGPu, followed by a stem-loop, and are basic structural modules critical for stability and functions of spliceosomal, telemorase and U7 RNPs. In Sm-class RNP assembly, assembly factor Gemin2 not only binds SmD1/D2/F/E/G (5Sm), but also serves as a checkpoint via a negative cooperativity mechanism: Gemin2 constricts the horseshoe-shaped 5Sm in a narrow conformation from outside, preventing non-cognate RNA and SmD3/B from joining; only cognate RNA can bind inside 5Sm and widen 5Sm, dissociating Gemin2 from 5Sm and recruiting SmD3/B. However, the structural mechanics is unknown. Here I describe a coordinate-improved structure of 5Sm bound by Gemin2/SMN. Moreover, via new analysis, comparison of this structure with those of newly coordinate-improved mature U1 and U4 Sm cores reveals how RNAs are selected, 5Sm conformation is changed, and Gemin2 is dissociated from 5Sm. Based on structure-guided sequence alignments, this assembly model is proposed to be conserved for all Sm cores in all eukaryotes. Finally, evolution of the assembly machinery is proposed and implications in spinal muscular atrophy are discussed.