Proper formation of different types of ribonucleoprotein (RNP)-RNA complexes is critical for the correct expression of genetic information at the posttranscriptional level. In the nucleus, RNA polymerase II transcripts bind to an abundant class of nuclear pre-mRNA/mRNA-binding proteins, the heterogeneous nuclear ribonucleoproteins (hnRNPs), to form hnRNP complexes (1 ,2 ). The molecular architecture of these complexes must be exceedingly dynamic because each step in the pre-mRNA processing pathway alters the structure of the underlying RNA substrate. Therefore, hnRNP complexes within the cell, although probably unique for each transcript, are not uniform structures, such as a ribosomal subunit or a particular small nuclear ribonucleoprotein particle (snRNP), but are composed of a heterogeneous mixture of protein-RNA structures.
