A NUCLEAR-LOCALIZATION DOMAIN IN THE HNRNP A1 PROTEIN

The heterogeneous nuclear RNP (hnRNP) A1 protein is one of the major pre-mRNA/mRNA binding proteins in eukaryotic cells and one of the most abundant proteins in the nucleus. It is localized to the nucleoplasm and it also shuttles between the nucleus and the cytoplasm. The amino acid sequence of A1 contains two RNP motif RNA-binding domains (RBDs) at the amino terminus and a glycine-rich domain at the carboxyl terminus. This configuration, designated 2 x RBD-Gly, is representative of perhaps the largest family of hnRNP proteins. Unlike most nuclear proteins characterized so far, A1 (and most 2 x RBD-Gly proteins) does not contain a recognizable nuclear localization signal (NLS). We have found that a segment of ca. 40 amino acids near the carboxyl end of the protein (designated MB) is necessary and sufficient for nuclear localization; attaching this segment to the bacterial protein beta-galactosidase or to pyruvate kinase completely localized these otherwise cytoplasmic proteins to the nucleus. The RBDs and another RNA binding motif found in the glycine-rich domain, the RGG box, are not required for A1 nuclear localization. M9 is a novel type of nuclear, localization domain as it does not contain sequences similar to classical basic-type NLS. Interestingly, sequences similar to MB are found in other nuclear RNA-binding proteins including hnRNP A2.