Exploring the architecture of the intact supraspliceosome using electron microscopy.

Splicing of pre-mRNA takes place on a massive macromolecular machine in the nucleus of eukaryotic cells, the supraspliceosome. This particle is a multicomponent biological complex of RNA and proteins. It is composed of four sub-structures termed native spliceosomes that splice pre-mRNA. The structure of the native spliceosome, determined by cryo-EM at 20 A resolution, showed that it is composed of two distinct subunits. Previously, medium resolution structural analysis of supraspliceosomes by electron tomography was performed, yet little is known of how the native spliceosomes are arranged within the intact particle. To address this question the native spliceosomes were analyzed and reconstructed in the context of the intact particle, using electron microscopy combined with image processing. Good correlation was obtained between the structure of the isolated native spliceosome, solved by cryo-EM, and the native spliceosome within the intact supraspliceosome. An ordered assembly was revealed with different potential roles assigned to the small and large subunits of the native spliceosome. The edges of the small subunits, which are in the center of the supraspliceosome, form a right angle and thus facilitate close contacts between the small subunits generating a 4-fold pattern. The analysis of sub-complex orientation within the particle suggests a possible route within the supraspliceosome for the passage of pre-mRNA, which is known to hold the particle together.