We analyzed the mechanism by which snRNAs are accumulated in the cell nucleus by introducing in vitro mutations into a cloned Xenopus U2 snRNA gene. The mutant genes were then expressed by microinjection into living oocytes. Using autoimmune antisera we localized the binding sites of snRNP proteins on the mutant U2 snRNAs. Sm antigen, a component shared by most U snRNPs, requires for binding a sequence containing AUUUUUG, a feature partly conserved in U1, U2, U4, and U5 snRNAs. A U2-specific protein defined by a second antiserum requires the two 3' loops of the U2 RNA molecule for binding. Mutant U2 transcripts unable to bind Sm antigen do not accumulate in the nucleus. Since Sm antigenic proteins are cytoplasmic and excluded from the oocyte nucleus when not bound to snRNA, we propose that a karyophilic domain may become exposed on formation of the RNA-protein complex.