The Heterogeneous Nuclear Ribonucleoprotein C Protein Tetramer Binds U1, U2, and U6 snRNAs through Its High Affinity RNA Binding Domain (the bZIP-like Motif)

Based on UV cross-linking experiments, it has been reported that the C protein tetramer of 40 S heterogeneous nuclear ribonucleoprotein complexes specifically interacts with stem-loop I of U2 small nuclear RNA (snRNA) (Temsamani, J., and Pederson, T. (1996) J. Biol. Chem. 271, 24922-24926), that C protein disrupts U4:U6 snRNA complexes (Forne, T., Rossi, F., Labourier, E., Antoine, E., Cathala, G., Brunel, C., and Tazi, J. (1995) J. Biol. Chem. 270, 16476-16481), that U6 snRNA may modulate C protein phosphorylation (Mayrand, S. H., Fung, P. A., and Pederson, T. (1996) Mol. Cell. Biol. 16, 1241-1246), and that hyperphosphorylated C protein lacks pre-mRNA binding activity. These findings suggest that snRNA-C protein interactions may function to recruit snRNA to, or displace C protein from, splice junctions. In this study, both equilibrium and non-equilibrium RNA binding assays reveal that purified native C protein binds U1, U2, and U6 snRNA with significant affinity ( approximately 7.5-50 nM) although nonspecifically. Competition binding assays reveal that U2 snRNA (the highest affinity snRNA substrate) is ineffective in C protein displacement from branch-point/splice junctions or as a competitor of C protein's self-cooperative RNA binding mode. Additionally, C protein binds snRNA through its high affinity bZLM and mutations in the RNA recognition motif at suggested RNA binding sites primarily affect protein oligomerization.