CLONING OF A COMPLEMENTARY-DNA ENCODING AN 80 KILODALTON NUCLEAR CAP-BINDING PROTEIN

It has been shown that the monomethylated cap structure plays importan t roles in nuclear events. The cap structure has been implicated in th e enhancement of pre-mRNA splicing. More recently, this structure has also been suggested to facilitate RNA transport from the nucleus to th e cytoplasm. We have previously identified and purified an 80kD Nuclea r Cap Binding Protein (NCBP) from a HeLa cell nuclear extract, which c ould possibly mediate these nuclear activities. In this report, we des cribe cloning of complementary DNA (cDNA) encoding NCBP. The partial p rotein sequences of NCBP were determined, and the full-length cDNA of NCBP was isolated from HeLa cDNA libraries. This cDNA encoded an open reading frame of 790 amino acids with a calculated molecular mass of 9 1,734 daltons, which contained most of the determined protein sequence s. However, the protein sequence had no significant homology to any kn own proteins. Transfection experiments demonstrated that the epitope-t agged NCBP, transiently expressed in HeLa cells, was localized exclusi vely in the nucleoplasm. Similar experiments using a truncated NCBP cD NA indicated that this nuclear localization activity is conferred by t he N-terminal 70 amino-acid region.