A nuclear matrix-associated factor, SAF-B, interacts with specific isoforms of AUF1/hnRNP D

One class of heterogeneous nuclear ribonucleoproteins (hnRNPs), AUF1/hnRNP D, consists of four isoform proteins (p45, p42, p40, and p37) which are generated by alternative splicing. The present study was therefore undertaken to clarify any isoform-specific differences in terms of their functions and nucleocytoplasmic localization. All isoforms primarily localized in the nucleus. However, heterokaryon analysis and a study using RNA polymerase II inhibitor revealed that p40/p37 exhibited a continuous shuttling between the nucleus and cytoplasm, Constant nuclear retention activity was mapped to the p45/p42-specific sequence at the C-terminal region, which is retained by alternative splicing. Using this domain as a probe, we performed a yeast two-hybrid screening and me found that scaffold attachment factor B (SAF-B), a nuclear matrix-associated protein, exhibits protein-protein interaction to this region, Colocalization of p45/p42 and SAF-B was observed as a speckle in the nucleus, Interestingly, p45/p42 isoforms appeared to act as a negative regulator in gene expression by forming a complex with SAF-B, Thus, the present study revealed that the isoform-specific functions of AUF1/hnRNP D are defined by intracellular shuttling capacity (C) 2000 Academic Press.