SnRNPs particles

Patients with the autoimmune disease systemic lupus erythematosus make autoantibodies directed against the Sm proteins of snRNP particles.552,553 Antibodies from different patients vary in tiieir specificities, making these antibodies a useful tool in the isolation and study of snRNAs and their protein complexes.552,554... 

Figure 28-22 Assembly and action of the spliceosomal complex. Four special sequence elements control the process the 5 and 3 splice sites, the branch point (adenosine A), and a polypyrimidine tract. The snRNP particle U1 locates the 5 splice site and U2 the branch point. The tri-snRNP U4 U6 U5 then binds, U6 recognizing the 5 splice site, and U1 and U4 are released. The 2 -OH of the branch point adenosine attacks the phos-phodiester linkage to form a lariet intermediate, which releases the intron in a lariet form in the final step. After Valcarcel and Green.612...

The spliceosome. The hnRNA of nuclei, which includes all of the pre-mRNA, is associated with proteins, which sometimes form very large 200S particles.604 After limited cleavage with nucleases they tend to sediment in the 30S-40S range and to contain a variety of proteins.605 606 Some of the proteins may have been involved in control of transcription.606 Others participate in splicing. The smaller snRNP particles then appear to come into the nucleus and displace much, but not all, of the protein present in the pre-mRNA ribonucleoprotein particles.605... 

Splicing takes place in a 50-60S small nuclear ribonucleoprotein (snRNP) particle called spliceosome consisting of a pre-mRNA, small nuclear RNAs (snRNAs) and a variety of pre-mRNA binding proteins, which ensures the proper excision of all introns and precise splicing of exons. 

The pure U snRNPs are modified using fluoroscein-A-hydroxysuccinimide ester (FLUOS, Boehringer Mannheim) and repurified on the antibody matrix to eliminate labeled proteins that are not stable components of the snRNP particle. 

Analysis of Purified snRNPs. The purity of the snRNP particles produced... 

Transport of RNA between the nucleus and cytoplasm is an essential step in the expression of messenger RNA (mRNA), and the function of ribosomal RNA (rRNA), transfer RNA (tRNA), and small nuclear RNA (snRNA). Most nuclear RNA transcripts are covalently modified, complexed with proteins into ribo-nucleoprotein particles (RNPs), and then exported through the nuclear pore complex to the cytoplasm. After export, most RNAs remain in the cytoplasm where they function in protein synthesis. However, many snRNAs are imported back to the nucleus as mature snRNP particles, where they function in pre-mRNA splicing. Small nucleolar RNAs (snoRNAs), which function in ribosome biogenesis, are not exported to the cytoplasm, but their intranuclear targeting to the nucleolus is an area of active research. 

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