The three mammalian ras proteins associated specifically with the plasma membrane and this is essential for their biological activity. Two signals encoded within the extreme COOH terminus of the proteins specify this cellular localization; a CAAX box in combination with either a polybasic domain (p21K-rasB) or a palmitoylation site (p21Ha-ras and p21N-ras). All members of the ras-like and rho-like subfamilies of the ras superfamily of small GTP-binding proteins also have CAAX boxes with potential second site sequences resembling either p21K-rasB or P21N-ras/Ha-ras. However it is not at all clear that they are each located at the plasma membrane, and in fact one of the ras-like proteins, rap1, has been localized to the Golgi (Beranger et al., 1991). None of the mammalian rho-like subfamily has yet been localized. Three forms (A, B, and C) of p21rho, the prototype of this family are known; the COOH termini of p21rhoA and p21rhoC resemble p21K-rasB with a polybasic domain, whereas p21rhoB resembles p21N-ras/Ha-ras with two cysteine residues as potential palmitoylation sites. Despite this similarity to the p21ras proteins, rho proteins have been purified from both particulate and cytosolic fractions of a variety of tissues. In order to localize definitively the three rho proteins we have used an epitope tagging approach coupled to microinjection of living cells. We show that a small fraction of all three proteins is localized to the plasma membrane but the majority of p21rhoA and p21rhoC is cytosolic whereas p21rhoB is associated with early endosomes and a pre-lysosomal compartment. Along with the results obtained with chimeric molecules using heterologous proteins attached to rho COOH termini, this suggests that the p21rho proteins cycle on and off the plasma membrane and this may have important implications for their biological function.

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