The NMR structure of the 38 kDa U1A protein - PIE RNA complex reveals the basis of cooperativity in regulation of polyadenylation by human U1A protein

The status of the poly(A) tail at the 3'-end of mRNAs controls the expressi on of numerous genes in response to developmental and extracellular signals . Poly(A) tail regulation requires cooperative binding of two human U1A pro teins to an RNA regulatory region called the polyadenylation inhibition ele ment (PIE). When bound to PIE RNA, U1A proteins also bind to the enzyme res ponsible for formation of the mature 3'-end of most eukaryotic mRNAs. poly( A) polymerase (PAP). The NMR structure of the 38 kDa complex formed between two U1A molecules and PIE RNA shows that binding cooperativity depends on helix C located at the end of the RNA-binding domain and just adjacent to t he PAP-interacting domain of U1A. Since helix C undergoes a conformational change upon RNA binding, the structure shows that binding cooperativity and interactions with PAP occur only when U1A is bound to its cognate RNA. Thi s mechanism ensures that the activity of PAP enzyme, which is essential to the cell, is only down regulated when U1A is bound to the U1A mRNA.