Necdin interacts with the ribonucleoprotein hnRNP U in the nuclear matrix

AbstractNecdin is expressed predominantly in terminally differentiated neurons, and its ectopic expression suppresses cell proliferation. We screened a cDNA library from neurally differentiated embryonal carcinoma P19 cells for necdin‐binding proteins by the yeast two‐hybrid assay. One of the positive clones contained cDNA encoding a carboxyl‐terminal portion of heterogeneous nuclear ribonucleoprotein U (hnRNP U), a nuclear matrix‐associated protein that interacts with chromosomal DNA. We isolated cDNA encoding full‐length mouse hnRNP U to analyze its physical and functional interactions with necdin. The necdin‐binding site of hnRNP U was located near a carboxyl‐terminal region that mediated the association between hnRNP U and the nuclear matrix. In postmitotic neurons, endogenously expressed necdin and hnRNP U were detected in the nuclear matrix and formed a stable complex. Ectopically expressed necdin was concentrated in the nucleoli, but coexpressed hnRNP U recruited necdin to the nucleoplasmic compartment of the nuclear matrix. Furthermore, under the same conditions necdin and hnRNP U cooperatively suppressed the colony formation of transfected SAOS‐2 cells. These results sugge