Germinal center-associated nuclear protein (GANP) has a phosphorylation-dependent DNA-primase activity that is up-regulated in germinal center regions

Antigen stimulation induces a rapid proliferation of B cells for expansion of specific B cell clones and their further differentiation into antibody-p roducing cells in germinal centers of T-dependent antigen-immunized mice. P reviously, we identified a 210-kDa germinal center-associated nuclear prote in (GANP) that is up-regulated selectively in germinal centers and carries an MCM-binding domain in the carboxyl-terminal side. In addition, here, we found a region (from 414 to 550 aa) in GANP molecule that is slightly simil ar to the known DNA-primase component p49. The recombinant GANP fragment co vering this region synthesizes RNA primers for extension by DNA polymerase I with single-stranded DNA templates in vitro. GANP DNA-primase activity is controlled by phosphorylation at Ser(502) that is induced by CD40-mediated signaling in vitro and in the germinal center B cells stimulated with anti gen in vivo. Overexpression of ganp cDNA in Daudi B cells caused the increa sed DNA synthesis more than the levels of the mock-transfectants. These evi dences suggested that the novel DNA-primase GANP is involved in regulation of cell proliferation of antigen-driven B cells in germinal centers.