The cleavage and polyadenylation specificity factor in Xenopus laevis oocytes is a cytoplasmic factor involved in regulated polyadenylation

During early development, specific mRNAs receive poly(A) in the cytoplasm. This cytoplasmic polyadenylation reaction correlates with, and in some case s causes, translational stimulation. Previously, it was suggested that a fa ctor similar to the multisubunit nuclear cleavage and polyadenylation speci ficity factor (CPSF) played a role in cytoplasmic polyadenylation. A cDNA e ncoding a cytoplasmic form of the 100-kDa subunit of Xenopus laevis CPSF ha s now been isolated. The protein product is 91% identical at the amino acid sequence level to nuclear CPSF isolated from Bos taurus thymus. This repor t provides three lines of evidence that implicate the X. laevis homologue o f the 100-kDa subunit of CPSF in the cytoplasmic polyadenylation reaction. First, the protein is predominantly localized to the cytoplasm of X. laevis oocytes. Second, the 100-kDa subunit of X. laevis CPSF forms a specific co mplex with RNAs that contain both a cytoplasmic polyadenylation element (CP E) and the polyadenylation element AAUAAA. Third, immunodepletion of the 10 0-kDa subunit of X. laevis CPSF reduces CPE-specific polyadenylation in vit ro. Further support for a cytoplasmic form of CPSF comes from evidence that a putative homologue of the 30-kDa subunit of nuclear CPSF is also localiz ed to the cytoplasm of X. laevis oocytes. Overexpression of influenza virus NS1 protein, which inhibits nuclear polyadenylation through an interaction with the 30-kDa subunit of nuclear CPSF, prevents cytoplasmic polyadenylat ion, suggesting that the cytoplasmic X. laevis form of the 30-kDa subunit o f CPSF is involved in this reaction. Together, these results indicate that a distinct, cytoplasmic form of CPSF is an integral component of the cytopl asmic polyadenylation machinery.