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The assembly of the spliceosome is a dynamic process that involves both small ribonucleoprotein particles (snRNPs), and non-snRNP proteins. The complete spliceosome consists of 5 snRNPs (U1, U2, U4, U5, and U6) and 50-100 non-snRNP factors. The snRNPs themselves contain a snRNA and a number of different proteins. These proteins can be subdivided into two classes, those specific to the individual snRNP, and those common to all snRNPs. The later group consists of eight polypeptides; B, B\', D1, D2, D3, E, F, and G, which bind together in a tight association, and are referred to as the sm proteins. While some of the particle-specific proteins have been well studied, i.e.U1-70K or U1-A, many have only been identified by their molecular weight and have yet to be cloned. These snRNP components are recruited to the pre-mRNA in an ordered manner, and begin with the recognition of exon-intron boundaries. The 5\' end of the U1 snRNP recognizes the 5\' splice site of the pre-mRNA. This interaction is mediated through conventional base pairing. Subsequently snRNP U1 recognizes and binds to the conserved branch point adenosine site. This step in the splicing cycle is called the commitment complex (CC) and is defined as a point where the formation of a stable splicing complex commits the spliceosome to assemble. Binding of the U1 snRNP encourages binding of U2 snRNP, and is helped by U2AF protein. SR proteins such as ASF/SF2 and SC35, or other intron bridging proteins, mediate the interaction between U1, U2 and U2AF. Binding of these factors facilitate recognition of the 3\' splice site, and this whole complex is called the E (Early) complex. Spliceosomal A Complex is formed following recruitment of U2 snRNP to the branch point sequence. Meanwhile, snRNPs U4/U6 and U5 have formed a functional tri-snRNP complex. The association of the tri-snRNP U4/U6-U5 with the pre-spliceosome creates the B complex. Several RNA:RNA interactions occur following this point, including the disassociation of the U4/U6 snRNP and the pairing of the 3\' end of U6 with the 5\' end of U2. These, and other, complex RNA rearrangements are required for mature spliceosomal formation, which is a pre requisite for the transesterification reactions that create the spliced mRNA. (This definition may be outdated - see the DesignNote.)
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