Mapping Nucleolar and Spliceosome Localization Sequences of Neuregulin1-beta3

Mitogenic growth factors are generally cell surface associated or secreted proteins, which produce effects by binding to cell surface receptor tyrosine kinases. More recently, it has become clear that some of these proteins can accumulate in the nucleus, where they are proposed to have transcriptional activity. We show here that neuregulin1 (NRG1-beta), an EGF-like growth factor, localizes to the cell nuclei of a human breast cancer. We also show that a nonsecreted isoform of this family of ligands, neuregulin1-beta3, localizes to two distinct compartments within the nucleus, nucleoli, and SC35-positive speckles. Importantly, localization of NRG-beta3 to either structure is receptor-independent, as it occurs in cells lacking its cognate receptors, erbB-3 and erbB-4, and is unaffected by removal of the receptor-binding domain. A panel of deletion mutants was used to demonstrate that the first 21 amino acids of the N-terminus are essential for nucleolar localization, while targeting to nuclear speckles requires residues 49-79 of the 241 amino acid protein. These observations support the idea that secretion and subsequent cell surface receptor binding of mitogenic growth factors are not a prerequisite for nuclear localization and that nonsecreted ligands may have highly specific functions in defined nuclear compartments.