Nuclear exchange of the U1 and U2 snRNP-specific proteins.

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Abstract
The snRNP particles include a set of common core snRPN proteins and snRNP specific proteins. In rodent cells the common core proteins are the B, D, D'', E, F and G proteins in a suggested stoichiometry of B2D2''D2EFG. The additional U1- and U2-specific proteins are the 70-kD, A and C proteins and the A'' and B'''' proteins, respectively. Previous cell fractionation and kinetic analysis demonstrated the snRNP core proteins are stored in the cytoplasm in large partially assembled snRNA-free intermediates that assemble with newly synthesized snRNAs during their transient appearance in the cytoplasm (Sauterer, R. A., R. J. Feeney, and G. W. Zieve. 1988. Exp. Cell Res. 176:344-359). This report investigates the assembly and intracellular distribution of the U1 and U2 snRNP-specific proteins. Cell enucleation and aqueous cell fraction are used to prepare nuclear and cytoplasmic fractions and the U1- and U2-specific proteins are identified by isotopic labeling and immunoprecipitation or by immunoblotting with specific autoimmune antisera. The A, C, and A'' proteins are found both assembled into mature nuclear snRNP particles and in unassembled pools in the nucleus that exchange with the assembled snRNP particles. The unassembled proteins leak from isolated nuclei prepared by detergent extraction. The unassembled A'' protein sediments at 4S-6S in structures that may be multimers. The 70-kD and B'''' proteins are fully assembled with snRNP particles which do not leak from isolated nuclei. The kinetic studies suggest that the B'''' protein assembles with the U2 particle in the cytoplasm before it enters the nucleus.