In the GTP bound state, Ras proteins activate multiple downstream effector proteins, the combination of which regulate cell growth, differentiation, and changes in cell shape. However, Ras proteins must undergo C-terminal lipidation before they can support biological activity. Specifically, the C-terminus of the HRas protein must be modified by a farnesyl at Cys186 and two palmitates at Cys181 and Cys184. The current paradigm suggests that palmitate is not attached without prior farnesyl addition and that these post-translational lipid modifications are only necessary for membrane association. However, emerging evidence suggests that the C-terminus and its lipid modifications may also be responsible for Ras mediated signal transduction and may represent a previously unsuspected mechanism to control either normal or malignant cell growth;Our studies with two novel mutants of HRas, ExtRas and G43:Ras, demonstrated that palmitoylation could occur in the absence of farnesylation, and begin to explore the role of palmitoylation and the C-terminus in interactions with effector proteins and membrane association. Specifically, the efficiency of membrane association via palmitoylation signals alone, either at the N-terminus (G43:HRas) or the C-terminus (ExtRas), is significantly less than that of a farnesyl, palmitate combination. These two novel HRas proteins also possess different biological activities despite the fact that they contain the same core Ras effector sequence (residues 32--40) and flanking residues which are important for interactions with all identified downstream effector proteins. Further analysis of the unusual biological activity Ext61L protein has found an imbalance in the activation of two specific Ras effector proteins which may underlie its unusual biological activity. These studies emphasize that palmitate and isoprenoid modifications have distinct structural and biological roles and point toward an unexpected participation of the HRas C-terminus in signal transduction.