Identification of heterogeneous nuclear ribonucleoprotein A/B as a cytoplasmic mRNA‐binding protein in early involution of the mouse mammary gland

Involution of the mammary gland is a regressive phase that occurs after lactation, and requires reprogramming of gene expression for the tissue to return to a pre‐pregnant state. Although the transcriptome of the mammary gland demonstrates complex changes at the mRNA level, the molecular mechanisms governing post‐transcriptional control remain obscure. In the present study, we isolated cytoplasmic mRNA‐protein complexes (mRNPs) from the mouse mammary gland at the early involution stage using discontinuous sucrose density ultracentrifugation. mRNPs including untranslated mRNAs were then purified with oligo(dT) immobilized on cellulose or paramagnetic beads. Proteins in the purified complexes were subjected to one/two‐dimensional gel electrophoresis followed by mass spectrometry. This identified heterogeneous nuclear ribonucleoprotein A/B (Hnrpab), along with three other heterogeneous nuclear ribonucleoproteins. Hnrpab in the mRNPs reproducibly increased within 48 h after weaning and became one of the major components. When a vector expressing Hnrpab was transfected into two different cell lines, their growth was suppressed, demonstrating that this protein has cytostatic activity. These results suggest that early involution can be used as a model for understanding the mechanism of post‐transcriptional control of gene expression, responsible for modulation of cell function. Copyright © 2010 John Wiley & Sons, Ltd.