Phosphorylated Ser/Arg-rich proteins: Limiting factors in the assembly of 200S large nuclear ribonucleoprotein particles

We have previously shown that specific nuclear pre-mRNA transcripts and their splicing products, as well as the general population of nuclear poly(A)⁺ RNA, are packaged in large nuclear ribonucleoprotein (lnRNP) particles that sediment at the 200S region in sucrose gradients. The lnRNP particles contain all uridine-rich small nuclear ribonucleoprotein complexes required for pre-mRNA splicing, as well as protein splicing factors. In this paper we show that all of the phosphorylated, mAb 104 detectable, Ser/Arg- rich essential splicing factors (SR proteins) in the nucleoplasm are integral components of the lnRNP particles, whereas only part of the essential splicing factor U2AF⁶⁵ (U2 snRNP auxiliary factor) and the polypyrimidine tract binding protein (PTB) are associated with these particles. This finding suggests a limiting role for SR proteins in the assembly of the lnRNP particles. We further show that the structural integrity of lnRNP particles is sensitive to variations in the phosphorylation levels of the SR proteins.