# Peptides That Block RAS-p21 Protein-Induced Cell Transformation
### Mitogenic Signal Transduction Pathways Induced by RAS-p21  
There are three major forms of RAS-p21: H (Harvey)-ras-, K (Kirsten)-ras- and N (neuroblastoma)-RAS-p21. H-RAS-p21 contains 189 amino acid residues (the others have a few more or less residues) and differ in sequence from one another, mostly at the carboxyl terminal end of the protein. All forms are G-proteins, i.e., they bind to GDP and become activated when they exchange GDP for GTP. This event must occur in the cell membrane where the RAS-p21 protein can interact with proteins described below that link RAS-p21 to growth factor receptors that span the cell membrane [1]. To induce the membrane localization of RAS-p21, the protein is farnesylated at Cys 186 wherein the -SH group of Cys 186 displaces the PPi moiety of farnesyl pyrophosphate catalyzed by the enzyme farnesyl transferase, a target for potential therapeutics. Cys 186 occurs on the carboxyl terminal end of RAS-p21 in the membrane localization \"CAAX box\" sequence of the form CAAX where C is Cys, A is a hydrophobic amino acid and X is any amino acid. Often, a Lys residue precedes the CAAX box. The activation of membrane-bound RAS-p21 involves several steps, as shown in Figure 1. In the first step, a growth factor, such as epidermal growth factor (EGF) or insulin, binds to its growth factor receptor, which contains three domains: the extracellular growth factor-binding domain, the transmembrane domain and the intracytosolic domain. Upon binding to its growth factor, the growth factor receptor dimerizes, leading to the activation of a tyrosine kinase domain on the cytosolic domain of the receptor. Upon activation, the tyrosine kinase domain phosphorylates tyrosine residues in the cytosolic domain of the receptor, allowing the receptor to bind to SH2 domains of adaptor proteins such as GRB2 [6], leading to the membrane localization of guanine exchange protein (GEF) or guanine nucleotide exchange proteins (GNEF). The latter two proteins bind to RAS-p21 and cause its activation by inducing the exchange of GDP for GTP. An important GNEF is called SOS (son of sevenless, a similar protein identified in Drosophila) [7]. Opposing these events, GTPase-Activating Proteins (GAP), a family of proteins, bind to the RAS-p21-GTP complex and induce inherent GTPase activity, resulting in the hydrolysis of GTP to GDP, presumably causing the complex to adopt an inactive form [8]. Figure 1: **[PERSON] et al. Wild-type (RAS-p21) and Oncogenic (RAS-p21+) Signal Transduction Pathways. ** A growth factor such as EGF or insulin binds to its receptor (upper left)—for example, EGFR—extracellularly. The receptor contains three domains: the extracellular growth factor-binding domain, the trans-membrane domain and the intracytosolic domain. The latter contains a tyrosine kinase (TK in the figure) which becomes activated when the receptors dimerize upon binding to the growth factor. This event enables the intracytosolic domain to bind to the adapter grb-2 protein that concurrently binds to and activates a guanine nucleotide exchange factor (GNEF), SOS, a protein that in turn binds to RAS-p21, causing it to exchange GDP for GTP. In the GTP-bound state, RAS-p21 becomes activated. In its activated state, it induces mitogenic signal transduction which occurs only if RAS-p21 is bound to the inner surface of the cell membrane through a covalently attached farnesyl moiety in thioether linkage to Cys 186 catalyzed by the enzyme, farnesyl transferase. If RAS-p21 is not membrane-bound, it acts as a competitive inhibitor of membrane-bound ras. An important target  
of activated membrane-bound RAS-p21 in mitogenic signaling is the serine-theonine kinase, raft, a 74 kDa Ser/Thr kinase protein that, in turn, directly binds to and activates another serine-threonine kinase, MEK in a so-called kinase cascade. The latter protein activates ERK.