The role of the leucine zipper domain in actin binding of p57 was investigated by the methods of the F-actin co-sedimentation studies in vitro, the immunofluorescence co-localization analyses in cells, the Western blot and density scan assays. The results showed that the leucine zipper-containing domain alone has no actin-binding activity, while the C-terminal deletion mutation removing this domain or the point mutation destroying leucine zipper domain leads to great reduction of actin-binding activity of p57. Furthermore, the degrees of degradation in actin-binding activity caused by the two mutations were assessed in quasi-quantitative analysis in vitro and in cells and both mutations exhibited the extremely similar depressing effects. These data strongly suggest that the leucine zipper domain is important for actin binding of p57.