The structure of the carboxyl terminus of the p21 protein. Structural relationship to the nucleotide-binding/transforming regions of the protein.

The carboxyl-terminal region of the ras oncogene-encoded p21 protein is critical to the protein's function, since membrane binding through the C-terminus is necessary for its cellular activity. X-ray crystal structures for truncated p21 proteins are available, but none of these include the C-terminal region of the protein (from residues 172-189). Using conformational energy analysis, we determined the preferred three-dimensional structures for this C-terminal octadecapeptide of the H-ras oncogene p21 protein and generated these structures onto the crystal structure of the remainder of the protein. The results indicate that, like other membrane-associated proteins, the membrane-binding C-terminus of p21 assumes a helical hairpin conformation. In several low-energy orientations, the C-terminal structure is in close proximity to other critical locales of p21. These include the central transforming region (around Gln 61) and the amino terminal transforming region (around Gly 12), indicating that extracellular signals can be transduced through the C-terminal helical hairpin to the effector regions of the protein. This finding is consistent with the results of recent genetic experiments.