Calmodulin binds to K-Ras, but not to H- or N-Ras, and modulates its downstream signaling

Activation of Ras induces a variety of cellular responses depending on the specific effector activated and the intensity and amplitude of this activat ion. We have previously shown that calmodulin is an essential molecule in t he down-regulation of the Ras/Raf/MEK/extracellularly regulated kinase (ERK ) pathway in cultured fibroblasts and that this is due at least in part to an inhibitory effect of calmodulin on Ras activation. Here we show that inh ibition of calmodulin synergizes with diverse stimuli (epidermal growth fac tor, platelet-derived growth factor, bombesin, or fetal bovine serum) to in duce ERK activation. Moreover, even in the absence of any added stimuli, ac tivation of Ras by calmodulin inhibition was observed. To identify the calm odulin-binding protein involved in this process, calmodulin affinity chroma tography was performed. We show that Ras and Raf from cellular lysates were able to bind to calmodulin. Furthermore, Ras binding to calmodulin was fav ored in lysates with large amounts of GTP-bound Ras, and it was Raf indepen dent. Interestingly, only one of the Ras isoforms, K-RasB, was able to bind to calmodulin. Furthermore, calmodulin inhibition preferentially activated K-Ras. Interaction between calmodulin and K-RasB is direct and is inhibite d by the calmodulin kinase II calmodulin-binding domain. Thus, GTP-bound K- RasB is a calmodulin-binding protein, and we suggest that this binding may be a key element in the modulation of Ras signaling.