Abstract
In .a wide variety of organisms, new ras-related genes have been identified on the basis of their sequence homologies with the mammalian H-ras, K-ras and N-ras proto-oncogenes. In mammals the proteins encoded by these ras-related genes have molecular weights of 21 000–24 000 daltons and share 30% to 50% homologies with the ras proteins (Chardin & Tavitian, 1986 ; Lowe et al. , 1987 ; Madaule & Axel, 1985 ; Touchot et al. , 1987) . By mutational analysis different regions of the ras proteins have been assigned to three functional domains: i) a carboxy-terminal region necessary for the anchorage of the protein to membranes, ii) an effector region (Willumsen et al., 1986; Sigal et al., 1986) that seems necessary for the interaction with the GTPase activating protein GAP (Trahey et al., 1987) and iii) a GTP binding domain (Barbacid, 1987). As ras transforming proteins detected in human tumors most frequently contain an amino acid substitution in the GTP binding domain, at either position 12, 13 or 61 (Barbacid, 1987) , this domain seems to play an essential role in the control of the biological properties of the protein.
Published Version
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