Localization of poly(A)-binding protein 2 (PABP2) in nuclear speckles is independent of import into the nucleus and requires binding to poly(A) RNA

The nuclei of mammalian cells contain domains, termed nuclear speckles, whi ch are enriched in splicing factors and poly(A) RNA. Although nuclear speck les are thought to represent reservoirs from which splicing factors are rec ruited to sites of transcription and splicing, the presence of poly(A) RNA in these structures remains enigmatic. An additional component of the speck les is poly(A) binding protein 2 (PABP2), a protein that binds with high af finity to nascent poly(A) tails, stimulating their extension and controllin g their length. In this work we investigated whether PABP2 contributes to t he targeting of poly(A) RNA to the speckles. The results show that localiza tion of PABP2 in speckles is independent of import of the protein into the nucleus. Inhibition of transcription or poly(A) synthesis at the end of mit osis does not affect nuclear import of PABP2 but prevents its localization to speckles. Furthermore, PABP2 mutants with decreased ability to bind to p oly(A) fail to localize to speckles. Taken together the results show that P ABP2 localizes to the nuclear speckles as a consequence of its binding to p oly(A) RNA, contrasting to splicing factors which assemble into speckles in the absence of newly synthesized transcripts.