Rat NAP1: cDNA cloning and upregulation by Mpl ligand

The Mpl ligand is a hematopoietic cytokine which exerts its effects through association with the c-Mpl receptor. It regulates the proliferation, polyploidization and maturation of platelet precursors, the megakaryocytes. Using a differential display polymerase chain reaction (PCR) approach, we have identified an mRNA, belonging to a family of nucleosome assembly proteins, whose expression is upregulated in response to Mpl ligand. Multiple size classes of this mRNA (1.7, 2.5 and 4.3 kb) are readily detected in rat primary bone marrow cells and hematopoietic tissues. The size classes are also expressed to different extents in cell lines of all hematopoietic lineages. We isolated the full-length cDNA encoding the rat megakaryocyte 1.7 kb mRNA, referred to as rNAP1. Bacterially expressed recombinant protein encoded by the 1.7 kb cDNA facilitates the formation of nucleosomes on relaxed circular DNA in vitro. Our data indicate that rNAPs, which may facilitate chromatin reorganization, are upregulated by Mpl ligand. It is possible that NAPs contribute to Mpl ligand's induced effects on hematopoietic cells.