Some of the proteins are common to all snRNPs and others are specific to individual snRNPs. The snRNPs assemble onto the RNA along with protein splicing factors to form a 60S ribonucleoprotein particle called the spliceosome, which is nearly as immense as the ribosome. Thus, much ATP is consumed during splicing despite the fact that the transesterification reactions that lie at the heart of the process are not energy requiring. This can occur in the coding region of a gene, thus giving rise to mRNAs with different coding sequences and ultimately different protein products. In some cases all of the different products are made in the same cell, but in other cases splicing is regulated such that different products with different properties are made in different cells.
Nuclear organization is maintained by a protein network in the nucleus called the nuclear matrix.
The mature mRNA is transported in the nucleus by an unknown mechanism to nuclear pores through which it passes to reach the cytoplasm. Some of the bound proteins are removed from the RNA during transport. Once in the cytoplasm, mRNAs are transported to specific locations or, more commonly, immediately bound by ribosomes and translated. Localization of mRNAs typically requires special sequences in the mRNA 3' untranslated region which are bound by specific binding proteins that probably associate with molecular motors that transport the mRNA to the specific cellular destination.