Chimeric proteins define variable and essential regions of Ha-ras-encoded protein.

The biological role of amino acid differences between the human 21-kDa Ha-ras protein (p21) and the human 23-kDa R-ras protein (p23) was investigated by engineering mutant Ha-ras p21 molecules containing divergent amino acid sequences from R-ras p23. Variant amino acids from R-ras p23 regions 1-30, 52-57, 67-78, 1-30 and 67-78 together, and 112-124 were substituted for the corresponding Ha-ras p21 amino acid regions 1-4, 26-31, 41-52, 1-4 and 41-52 together, and 86-98, respectively. Rat fibroblasts transfected with genes encoding these position-12 valine-substituted chimeric Ha-ras proteins displayed the same properties of morphological transformation and anchorage-independent growth as Ha-ras T24 oncogene-transformed fibroblasts. However, substitution of variant amino acids from the 80 C-terminal residues (amino acids 138-218) of R-ras p23 for the corresponding p21 amino acids (residues 112-189) inactivated the transforming activity of position-12 valine-substituted p21. The converse substitution of Ha-ras p21 C-terminal residues into R-ras p23 did not result in transformation by position-38 valine-substituted p23. These data are discussed in terms of the structure of ras proteins and the nature of interactions determining the specificity of effector function.