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Crystallization and biophysical characterization of spliceosomal protein complexes

mRNA splicing is a highly dynamic process by which non-coding sequences are 
removed from a pre-mature mRNA. It is catalyzed by the spliceosome, a huge and 
highly dynamic ribonucleoprotein complex consisting of up to five small nuclear 
RNAs and more than 100 proteins. For each intron to be excised, the spliceosome 
has to assemble on the pre-mRNA, catalyze the two steps of the splicing 
reaction and is disassembled afterwards. The dynamics of the spliceosome are 
driven and controlled by at least eight conserved DExD/H-box ATPases which 
remodel RNA-RNA, protein-RNA and protein-protein interactions during assembly, 
catalysis and disassembly of the spliceosome and are furthermore involved in 
quality control. 
The DEAH-box ATPase Prp2 plays a key role in the activation of the spliceosome, 
as it promotes the transition of the spliceosome from the Bact to the catalytically 
active B* complex. Prp2 exhibits an RNA-dependent NTPase activity and 
is active in the spliceosome after binding to its interaction partner, the G-patch 
containing protein Spp2. The G-patch sequence motif consists of a conserved 
glycine pattern and is mostly found in proteins involved in RNA processing. So 
far, the structure of a G-patch motif has been unknown and the function of the 
G-patch motif in general and in particular of Spp2 remained elusive. 
Here, the crystal structure of Spp2 from Chaetomium thermophilum in complex 
with Prp2 is shown, representing the first crystal structure of a G-patch protein. 
Free Spp2 is intrinsically disordered as demonstrated by CD- and NMR- spectroscopy, 
but at least its G-patch region adopts a well-defined fold upon 
binding to Prp2. The interaction with Spp2 induces major structural changes in 
the beta-hairpin of Prp2, a region which is supposed to move upon ATP binding. 
However, Spp2 does not affect the ATPase activity or RNA affinity of Prp2 in 
vitro. Together, these findings give rise to the hypothesis that Spp2 might fulfill 
the function of an adaptor protein recruiting Prp2 to the spliceosome and transferring 
mechanical forces generated by ATP hydrolysis to other components of 
the spliceosome.