U1A Inhibits Cleavage at the Immunoglobulin M Heavy-Chain Secretory Poly(A) Site by Binding between the Two Downstream GU-Rich Regions

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Abstract
The immunoglobulin M heavy-chain locus contains two poly(A) sites which are alternatively expressed during B-cell differentiation. Despite its promoter proximal location, the secretory poly(A) site is not expressed in undifferentiated cells. Crucial to the activation of the secretory poly(A) site during B-cell differentiation are changes in the binding of cleavage stimulatory factor 64K to GU-rich elements downstream of the poly(A) site. What regulates this change is not understood. The secretory poly(A) site contains two downstream GU-rich regions separated by a 29-nucleotide sequence. Both GU-rich regions are necessary for binding of the specific cleavage-polyadenylation complex. We demonstrate here that U1A binds two (AUGCN1-3C) motifs within the 29-nucleotide sequence and inhibits the binding of cleavage stimulatory factor 64K and cleavage at the secretory poly(A) site.