Killin functions in p53 mediated S phase arrest and apoptosis in the absence of p21, puma and bax were reported. Through co transfection of EGFP PCNA and RFP killin plasmids into cosE5 cells, It was shown that Killin and PCNA exhibit in mutually exclusive pattern in the nucleus during S phase. Consistent with previous finding using BrdU labeling, these results confirm that Killin may function in S phase checkpoint control. Surprisingly, we also observe that Killin localizes mostly in cell nucleolus in non S phase cells, suggesting a potential role of Killin in negative regulation of rRNA biogenesis. Thus, as a major p53 target gene, killin may mediate p53 functions in multiple cell cycle checkpoints.
