Identification of amino acids in p21ras involved in exchange factor interaction.

Through the genetic analysis of vulva development in C. elegans several different sites of mutation have been identified in the let-60 ras protein which have been postulated to affect the function of normal but not oncogenic p21ras (Beitel, G. J., S. G. Clark, and H. R. Horvitz. 1990. Nature 348: 503-509). We have introduced these mutations into mammalian Ha-ras and determined their effect on the function of cellular ras (c-ras), an oncogenically activated variant D12ras and the dominant negative N17ras. From these studies we conclude that two of the mutations S89-->F89 and delta 103-108 destabilise ras when it is in the GDP-bound form. However mutations at A66 and G75 lead to stable proteins on which a ras exchange factor SCD25 is unable to promote the formation of ras-GTP. The mutations at A66 show for the first time that helix alpha 2 of p21ras is involved in the stimulation of guanine nucleotide exchange by exchange factors.