Mdm2-Mdmx exchanges at the N terminus of the RING finger prevent complete p53 ubiquitylation. (A) Schematic representation of the Mdmx-Mdm2 chimeras with exchanges at the N terminus of the RING or containing the complete Mdm2 RING. (B and C) p53- and mdm2-null cells (B) and H1299 cells (C) were transfected in duplicate with the indicated constructs; ubiquitylated proteins and total lysates were analyzed as described for Fig. 2B. (D) Immunofluorescence analysis of C33A cells transfected with 22(x-2)A, 22(x-2)A N447C, or 22(x-2)Afl. Cells were stained for expression of the chimeric proteins with 4B2 (a, d, and g) and for expression of p53 with FL393 (b, e, and h). Nuclei were stained with 4′,6′-diamidino-2-phenylindole (DAPI) (c, f, and i).