Retention of p63 in an ER-Golgi intermediate compartment depends on the presence of all three of its domains and on its ability to form oligomers

The endogenous p63 could be visualized in saponin-permeabilized COS cells by indirect immunofluorescence using a 1:20 dilution of mAb G1/296 which is specific for this protein. As shown in Fig. 1 a, the fluorescence pattern with its extended ER-Golgi intermediate membrane structure was very similar to that observed previously with Vero cells (Schweizer et al., 1993a) . While ER-like, the pattern differs from that observed with ER markers in that it is less reticular and the outer nuclear membrane does not stain (see Fig. 2 c for a typical ER pattern and Schweizer et al., 1993a) . When the mAb G1/296 was tested at a dilution of 1:2,000, the p63 staining pattern was very faint (Fig. 1 b) . Since this dilution of antibody gave a strong signal with p63 transfected COS cells (Fig. 2 a) , it was possible to readily distinguish between endogenous and transfected p63. COS cells transfectext with IRi3wt exhibited the typical extended membrane structure observed when endogenous p63 was stained ( Fig. 2 a, left cell). In addition, some of the cells showed a large tubular network pattern that was not observed in the untransfected COS cells (Fig. 2 a, right cell) (Schweizer et al., 1993b) . P63wt was never detected at the cell surface, even when the expression level was very high (Fig. 2 b) .