Cell cycle-dependent colocalization of BARD1 and BRCA1 proteins in discrete nuclear domains

Germ-line mutations of the BRCA1 gene predispose women to early onset breast and ovarian cancer by compromising the gene's presumptive function as a tumor suppressor, Although the biochemical properties of BRCA1 polypeptides are not understood, their expression pattern and subcellular localization suggest a role in cell-cycle regulation, When resting cells are induced to proliferate, the steady-state levels of BRCA1 increase in late G(1) and reach a maximum during S phase. Moreover, in S phase cells, BRCA1 polypeptides are hyperphosphorylated and accumulate into discrete subnuclear foci termed ''BRCA1 nuclear dots.'' BRCA1 associates in vivo with a structurally related protein termed BARD1, Here we show that the steady-state levels of BARD1, unlike those of BRCA1, remain relatively constant during cell cycle progression, However, immunostaining revealed that BARD1 resides within BRCA1 nuclear dots during S phase of the cell cycle, but not during the G(1) phase, Nevertheless, BARD1 polypeptides are found exclusively in the nuclear fractions of both G(1)- and S-phase cells, Therefore, progression to S phase is accompanied by the aggregation of nuclear BARD1 polypeptides into BRCA1 nuclear dots, This cell cycle-dependent colocalization of BARD1 and BRCA1 indicates a role for BARD1 in BRCA1-mediated tumor suppression.