Farnesylated CaaX protein

Fig. 4.3. Farnesylated CaaX proteins are more dependent on carboxylmethylation by Icmt for correct membrane localization than are geranylgeranylated CaaX proteins. (A) The indicated GFP-tagged Ras isoform was expressed in MEFs deficient in Icmt (-/-) or wild-type MEFs from httermates (+/+). All three Ras isoforms showed marked mislocalization with accumulation of GFP-Ras in the fluid phase cytosol. (B) GFP-tagged Rho proteins were expressed in the same MEFs as in (A). Activated forms of Racl, RhoA, and Cdc42 were utilized for clarity since these forms do not bind to the cytosolic chaperone RhoGDI but instead associate with the cellular membranes to which their C-termini are targeted. In contrast to farnesylated Ras proteins, geranylgeranylated Rho proteins are localized with similar patterns in both +/+ and -/- MEFs.

The structure and mechanism of catalysis of FTase were well defined in the late 1990s from several X-ray crystallography and elegant biochemical studies [24,26-30]. The enzyme is a heterodimer of a and P subunits [31,32]. The P subunit contains binding sites for both the farnesyl pyrophosphate and the CAAX protein substrates. A catalytic zinc (Zn " ) identified in the active site of the P subunit participates in the binding and activation of the CAAX protein substrates [28]. The Zn " is coordinated to the enzyme in a distorted tetrahedral geometry and surrounded by hydrophobic pockets [24,27]. Upon binding of the CAAX peptide, the thiol of the cysteine displaces water and is activated for a nucleophilic attack via thiolate on the C-1 carbon atom of farnesyl pyrophosphate [30]. 

Following geranylgeranylation or farnesylation, modified CAAX proteins undergo proteolytic cleavage of the AAX, followed by carboxy O methyla-tion [33,34]. The Ras CAAX endoprotease or RCE is currently the target of several drug discovery programs. All three reactions are required for full activation or membrane localization of the proteins. 

To confirm further that inhibition of farnesylation of GFP-CaaX-proteins is because of the binding of the molecular forceps to their CaaX-sequence, and not because of inhibition of FTase, we performed in-vitro binding assays with FTase. Whereas MF3 clearly interacts with H-RAS at a concentration of 250 gM, two and a half times the observed IC50, we did not observe binding to Ftase (Figure 3.1.4b). Because the forceps bind to RAS and fail to bind FTase, we concluded that MF3 could not act as an enzyme inhibitor, but that its activity is caused by its interaction with the substrate [26]. 

Fig. 10.1. Schematic of CaaX protein biosynthesis. A precursor containing a CaaX motif is sequentially modified to yield a product with an isoprenylated and carboxylmethylated C-ter-minus. Rcelp is specific for Ras substrates and partially redundant with Ste24p in mediating proteolysis of certain other CaaX proteins (step 2). FTI, farnesyl transferase inhibitor RPI, Rcelp protease inhibitor.

Outside of yeast, evidence for the importance of CAAX protein methylation was more difficult to come by, due to the fact that the farnesylated and proteolyzed Ras would be methylated immediately making it difficult to assess the functional impact of methylation [23]. Most evidence before the mammalian genes were identified and disrupted came from inhibition of this methylation step by using substrate mimics such as A-acetyl-5-famesylcysteine (AFC). 

Fig. 2. Famesylation, proteolysis, and carboxymethylation of a CaaX protein. The farnesyl donor is farnesyl pyrophosphate. FTase, famesyltransferase Rce, CaaX protease Icmt, methyltransferase.

FIGURE 9.19 Proteins containing the C-terminal sequence CAAX can undergo prenylation reactions that place thioether-linked farnesyl or geranylgeranyl groups at the cysteine side chain. Prenylation is accompanied by removal of the AAX peptide and methylation of the carboxyl group of the cysteine residue, which has become the C-terminal residue. 

Protein farnesyl transferase also can be used to add a geranylazide derivative to a synthetic peptide by incorporating the enzyme recognition sequence CAAX at the C-terminal of any... 

The inositol polyphosphate 5-phosphatases belong to a family of enzymes that terminate the signals generated by inositol lipid kinases and PLC. To date, two major types of 5-phosphatase have been identified, both of which share a common 5-phosphatase domain of approximately 300 amino acids, with several highly conserved motifs. Type-I enzymes are 43-65 kDa and preferentially hydrolyze 1(1,4,5)P3 and 1(1,3,4,5)P4, with the attendant formation of I(1,4)P2 and 1(1,3,4)P3, but have little or no activity towards membrane-bound phosphoinositides. The pro-totypic form of a type-15-phosphatase is a 43 kDa protein that is post-translationally modified by farnesylation of the carboxyl terminus CAAX motif this modification juxtaposes the enzyme with the membrane. Type-II enzymes are larger (75-160 kDa) and will hydrolyze both water-soluble inositol phosphates and lipids that... 

The Ras proteins are synthesized as biologically inactive, cytosolic precursor proteins. They are then modified by several post-translational processing steps at the carboxyl terminal end and thereby converted into biologically active proteins localized at the plasma membrane. The cysteine of the C-terminal CAAX sequence (C is cysteine, A is generally an aliphatic amino acid, and X is methionine, serine, alanine, or glutamine) is first enzymatically S-farnesylated the AAX part is then cleaved off by a specific protease, and the free C-terminal cysteine is finally converted into a methyl ester (Scheme 1). 

Although FTase inhibitors influence the farnesylation of Ras they are likely to interfere with the posttranslational modifications of other CAAX-containing proteins as well. Apart from the approximately 20 farnesylated proteins that are known today, farnesylation is also required for normal Ras function which in turn is critical for normal cell viability. For these reasons farnesyltransferase... 

In normal cells, the GDP/GTP-binding proteins, after protein synthesis, move to the cell membrane to which they become hooked by a hydrophobic farnesyl group. The y-subunit is anchored in the membrane by a post-translational modification of the C-terminal CAAX sequence (C - cystein, AA - aliphatic amino acids, X - methionine). This protein is first enzymatically farnesylated by a specific farnesyltransferase, then the AAX part is cleaved by specific proteases and finally the cystein residue is converted to a methyl ester. 

The molecular elements of that pathway were mapped with photoaffinity labeling by different investigators. Farnesyltransferase contains a and heterodimer subunits, and binds to both protein and farnesyl diphosphate. The main recognition elements for the protein is the C-terminal CAAX motif. Coleman et al. attached two benzophenones to the recognition sequence and the resulting photoprobe (38, Fig. 14) specifically labeled both subunits [125]. 

Studying the sequences of farnesylated proteins indicated that all lipidated proteins bear a cysteine residue near the C-terminus revealing the CAAX-motif, where C is a cysteine, A stands for an aliphatic amino acid, and X can be any amino acid. Database searches resulted in more prenylated proteins, all bearing the CAAX-motif, in systems from lower eukaryotes to mammals. A closer look at the mature proteins revealed that prenylation was only the first step of processing of the CAAX-motif-encoded proteins. After transfer of the isoprene unit, the last three amino acids are cleaved proteolytically by an endoprotease and the C-terminal cysteine is carboxymethylated by a methyltransferase. ... 

Proteins can undergo different rounds of palmitoylation and depalmitoylation, either constitutively or as a response to signals." " Here the Ras proteins are the most commonly discussed examples. As described above, all Ras proteins are expressed with the CAAX-box and are subject to post-translational modifications. First, they get farnesylated and after proteolysis and methylation of the C-terminus, H-/N-Ras as well as K-Ras 4A get further palmitoylated at additional cysteines present in their C-terminus. Palmitoylation occurs in the Golgi apparatus and via vesicular transport the farnesylated and palmitoylated proteins are directed to the plasma membrane (PM). The palmitoyl thioester is hydrolyzed at multiple cellular sites and the protein is transported back to the Golgi via a nonvesicular pathway (Scheme 3)." ... 

FTase catalyzes the covalent attachment of a farnesyl moiety via a thioether Unkage to the proteins bearing a C-terminal amino acid sequence known as the CAAX motif (Fig. 2) [12,21]. The farnesyl moiety is derived from farnesyl pyrophosphate (FPP), a 15-carbon isoprenyl intermediate in the mevalonate pathway of cholesterol biosynthesis. The binding of FPP to the enzyme has relatively high affinity (K = 1-lOnM), and FPP binding must precede the binding of the peptide substrate for successful catalysis [22,23]. 

Fig. 4 CAAX competitive heterocyclic farnesyl protein transferase inhibitors. The inhibitors shown in the first row are reported to be in phase I, II, or III clinical development...

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