Endomembrane trafficking of Ras: The CAAX motif targets proteins to the ERand Golgi

We show that Nras is transiently localized in the Golgi prior to the plasma membrane (PM). Moreover, green fluorescent protein (GFP)-tagged Nras illum inated motile, peri-Golgi vesicles, and prolonged BFA treatment blocked PM expression. GFP-Hras colocalized with GFP-Nras, but GFP-Kras4B revealed les s Golgi and no vesicular fluorescence. Whereas a secondary membrane targeti ng signal was required for PM expression, the CAAX motif alone was necessar y and sufficient to target proteins to the endomembrane where they were met hylated, a modification required for efficient membrane association. Thus, prenylated CAAX proteins do not associate directly with the PM but instead associate with the endomembrane and are subsequently transported to the PM, a process that requires a secondary targeting motif.