The structure of U1 snRNP. Each snRNP consists of an snRNA (U1 is shown here), with an Sm-binding site, as well as unique sequences and secondary structures. In addition, proteins unique to each snRNA bind to form a complex (the U1 snRNP proteins U1 A, U1 C, and U1 70K are shown here). (a) Two-dimensional and (b) three-dimensional representations of the U1 snRNP. Sm proteins bind to the Sm-binding site to form the Sm core domain; unique proteins bind elsewhere in the snRNA to form the complete snRNP.