Protein farnesyltransferase: structure and implications for substrate binding

Non-peptidic, non-prenylic bisubstrate farnesyltransferase inhibitors. Part 3: structural requirements of the central moiety for farnesyltransferase inhibitory activity

Recently, we have described non-peptidic, non-prenylic bisubstrate analogues as a novel type of farnesyltransferase inhibitor composed of a farnesyl-mimetic, a linker and an AAX-peptidomimetic substructure. With this study, we showed that the amide function connecting the farnesyl-mimetic and the linking substructures of our inhibitors is crucial for their activity. We suggest that the amide is bound to the essential zinc ion in the farnesyltransferases active center. We identified succinic and glutaric acid, respectively, in addition to the initially used 1-alanyl moiety as suitable linking structures. Glycine can also be used in this function provided the distance between the alpha-amide group and the center of the peptidomimetic substructure is enlarged by introduction of an additional methylene unit into the peptidomimetic substructure.

Design and synthesis of a transferable farnesyl pyrophosphate analogue to Ras by protein farnesyltransferase

The posttranslational addition of a farnesyl moiety to the Ras oncoprotein is essential for its membrane localization and is required for both its biological activity and ability to induce malignant transformation. We describe the design and synthesis of a farnesyl pyrophosphate (FPP) analogue, 8-anilinogeranyl pyrophosphate 3 (AGPP), in which the omega-terminal isoprene unit of the farnesyl group has been replaced with an aniline functionality. The key steps in the synthesis are the reductive amination of the alpha,beta-unsaturated aldehyde 5 to form the lipid analogue 6, and the subsequent conversion of the allylic alcohol 7 to the chloride 8 via Ph(3)PCl(2) followed by displacement with [(n-Bu)(4)N](3)HP(2)O(7) to give AGPP (3). AGPP is a substrate for protein farnesyltransferase (FTase) and is transferred to Ras by FTase with the same kinetics as the natural substrate, FPP. AGPP is highly selective, showing little inhibitory activity against either geranylgeranyl-protein transferase type I (GGTase I) (K(i) = 0.06 microM, IC(50) = 20 microM) or squalene synthase (IC(50) = 1000 microM). AGPP is the first efficiently transferable analogue of FPP to be modified at the omega-terminus that provides a platform from which additional analogues can be made to probe the biological function of protein farnesylation. AGPP is the first example of a class of compounds that are alternate substrates for protein isoprenylation that are not inhibitors of squalene synthase.

Crystal structure of Rab geranylgeranyltransferase at 2.0 A resolution

BACKGROUND

Rab geranylgeranyltransferase (RabGGT) catalyzes the addition of two geranylgeranyl groups to the C-terminal cysteine residues of Rab proteins, which is crucial for membrane association and function of these proteins in intracellular vesicular trafficking. Unlike protein farnesyltransferase (FT) and type I geranylgeranyltransferase, which both prenylate monomeric small G proteins or short peptides, RabGGT can prenylate Rab only when Rab is in a complex with Rab escort protein (REP).

RESULTS

The crystal structure of rat RabGGT at 2.0 A resolution reveals an assembly of four distinct structural modules. The beta subunit forms an alpha-alpha barrel that contains most of the residues in the active site. The alpha subunit consists of a helical domain, an immunoglobulin (Ig)-like domain, and a leucine-rich repeat (LRR) domain. The N-terminal region of the alpha subunit binds to the active site in the beta subunit; residue His2alpha directly coordinates a zinc ion. The prenyl-binding pocket of RabGGT is deeper than that in FT.

CONCLUSIONS

LRR and Ig domains are often involved in protein-protein interactions; in RabGGT they might participate in the recognition and binding of REP. The binding of the N-terminal peptide of the alpha subunit to the active site suggests an autoinhibition mechanism that might contribute to the inability of RabGGT to recognize short peptides or Rab alone as its substrate. Replacement of residues Trp102beta and Tyr154beta in FT by Ser48beta and Leu99beta, respectively, in RabGGT largely determine the different lipid-binding specificities of the two enzymes.

Farnesyl protein transferase inhibitors and other therapies targeting the Ras signal transduction pathway

The year 2000 will be a significant date for the field of Ras-related therapies since numerous agents will have Phase II clinical efficacy data maturing to provide proof of principle for this cancer treatment strategy. These data will also provide an important milestone for the cancer research community since these molecules represent a small vanguard of oncology drug discovery projects predicated on molecular targets. We can only hope that these agents are a successful harbinger for the formidable number of targeted therapies that will be entering development pipelines in the coming years.

The basis for K-Ras4B binding specificity to protein farnesyltransferase revealed by 2 A resolution ternary complex structures

BACKGROUND

The protein farnesyltransferase (FTase) catalyzes addition of the hydrophobic farnesyl isoprenoid to a cysteine residue fourth from the C terminus of several protein acceptors that are essential for cellular signal transduction such as Ras and Rho. This addition is necessary for the biological function of the modified proteins. The majority of Ras-related human cancers are associated with oncogenic variants of K-RasB, which is the highest affinity natural substrate of FTase. Inhibition of FTase causes regression of Ras-mediated tumors in animal models.

RESULTS

We present four ternary complexes of rat FTase co-crystallized with farnesyl diphosphate analogs and K-Ras4B peptide substrates. The Ca(1)a(2)X portion of the peptide substrate binds in an extended conformation in the hydrophobic cavity of FTase and coordinates the active site zinc ion. These complexes offer the first view of the polybasic region of the K-Ras4B peptide substrate, which confers the major enhancement of affinity of this substrate. The polybasic region forms a type I beta turn and binds along the rim of the hydrophobic cavity. Removal of the catalytically essential zinc ion results in a dramatically different peptide conformation in which the Ca(1)a(2)X motif adopts a beta turn. A manganese ion binds to the diphosphate mimic of the farnesyl diphosphate analog.

CONCLUSIONS

These ternary complexes provide new insight into the molecular basis of peptide substrate specificity, and further define the roles of zinc and magnesium in the prenyltransferase reaction. Zinc is essential for productive Ca(1)a(2)X peptide binding, suggesting that the beta-turn conformation identified in previous nuclear magnetic resonance (NMR) studies reflects a state in which the cysteine is not coordinated to the zinc ion. The structural information presented here should facilitate structure-based design and optimization of inhibitors of Ca(1)a(2)X protein prenyltransferases.

Successful virtual screening of a chemical database for farnesyltransferase inhibitor leads

Virtual screening of chemical databases is an emerging approach in drug discovery that uses computers to dock chemicals into the active site of a drug target to identify leads through evaluation of binding affinities of the chemicals. However, there are concerns about the validity and scope of the reported virtual screens due to lack of studies to show that randomly selected chemicals are not equally active and due to the fact that metalloproteins were rarely used as drug targets. We have performed a virtual screening of a chemical database to identify prototypic inhibitors of farnesyltransferase (FT) with zinc present in the active site. Among the 21 compounds identified by computers, four inhibited FT in vitro with IC(50) values in the range from 25 to 100 microM. The most potent inhibitor also inhibited FT in human lung cancer cells. In contrast, none of 21 randomly selected compounds have an IC(50) lower than 100 microM. The results demonstrate the validity of virtual screening and the feasibility of applications of this approach to metalloprotein drug targets, such as matrix metalloproteinases, farnesyltransferase, and HIV-1 integrase, for the treatments of cardiovascular diseases, cancers, and AIDS.

Yeast protein farnesyltransferase. pKas of peptide substrates bound as zinc thiolates

Protein farnesyltransferase (PFTase) is a zinc metalloenzyme that catalyzes the posttranslational alkylation of the cysteine in C-terminal -Ca(1)a(2)X sequences by a 15-carbon farnesyl residue, where C is cysteine, a(1) and a(2) are normally aliphatic amino acids, and X is an amino acid that specifies selectivity for the farnesyl moiety. Formation of a Zn(2+) thiolate in the PFTase. peptide complex was detected by the appearance of an absorbance at 236 nm (epsilon = 15 000 M(-1) cm(-1)), which was dependent on the concentration of peptide, in a UV difference spectrum in a solution of PFTase and the peptide substrate RTRCVIA. We developed a fluorescence anisotropy binding assay to measure the dissociation constants as a function of pH for peptide analogues by appending a 2',7'-difluorofluorescein to their N-terminus. The electron-withdrawing fluorine atoms allowed us to measure peptide binding down to pH 5.5 without having to correct for the changes in fluorescence intensity that accompany protonation of the fluorophore. Measurements of the pK(a)s for thiol groups in free and bound peptide indicate that peptide binding is accompanied by formation of a zinc thiolate and that binding to PFTase lowers the pK of the peptide thiol by 3 units. In similar studies with the betaY310F mutant, the pK(a) of the thiol moiety was lowered by 2 units upon binding, indicating that the hydroxyl group in the conserved tyrosine helps stabilize the bound thiolate.

Characterization of the ternary complex between Rab7, REP-1 and Rab geranylgeranyl transferase

Geranylgeranylation is a post-translational modification of Rab GTPases that enables them to associate reversibly with intracellular membranes. Geranylgeranylation of Rab proteins is critical for their activity in controlling intracellular membrane transport. According to the currently accepted model for their action, newly synthesized Rab proteins are recruited by Rab escort protein (REP) and are presented to the Rab geranylgeranyl transferase (RabGGTase) which covalentely modifies the Rab protein with two geranylgeranyl moieties. After prenylation, the Rab protein remains in complex with REP and is delivered to the target membrane by the latter. In this work, we show that RabGGTase can form a stable complex with Rab7-REP in the absence of its lipid substrate geranylgeranyl pyrophosphate. In order to characterize this interaction, we developed three fluorescence assays reporting on the interaction of RabGGTase with the Rab7-REP complex. For this interaction we determined a Kd value of about 120 nM. Association of RabGGTase with the Rab7-REP complex occurs with a rate constant of approximately 108 M-1 x s-1. We demonstrate that the state of the nucleotide bound to Rab7 does not influence the affinity of RabGGTase for the Rab7-REP-1 complex. Finally, we address the issue of substrate specificity of RabGGTase. Titration experiments demonstrate that, in contrast with farnesyl transferase, RabGGTase does not recognize a defined C-terminal sequence motif. Experiments using Rab7 mutants in which the last 16 amino acids were either mutated or truncated revealed that the distal part of the C-terminus makes only a limited contribution to the binding affinity between RabGGTase and the Rab7-REP-1 complex. This demonstrates the functional dissimilarity between RabGGTase and geranylgeranyl transferase I and farnesyl transferase, which interact specifically with the C-terminus of their substrates. Based on these experiments, we propose that RabGGTase recognizes the overall structure arising from the association of Rab and REP and then 'scans' the flexible C-terminus to position the proximal cysteines into the active site.

A mutant form of human protein farnesyltransferase exhibits increased resistance to farnesyltransferase inhibitors

Protein farnesyltransferase (FTase) is a key enzyme responsible for the lipid modification of a large and important number of proteins including Ras. Recent demonstrations that inhibitors of this enzyme block the growth of a variety of human tumors point to the importance of this enzyme in human tumor formation. In this paper, we report that a mutant form of human FTase, Y361L, exhibits increased resistance to farnesyltransferase inhibitors, particularly a tricyclic compound, SCH56582, which is a competitive inhibitor of FTase with respect to the CAAX (where C is cysteine, A is an aliphatic amino acid, and X is the C-terminal residue that is preferentially serine, cysteine, methionine, glutamine or alanine) substrates. The Y361L mutant maintains FTase activity toward substrates ending with CIIS. However, the mutant also exhibits an increased affinity for peptides terminating with CIIL, a motif that is recognized by geranylgeranyltransferase I (GGTase I). The Y361L mutant also demonstrates activity with Ha-Ras and Cdc42Hs proteins, substrates of FTase and GGTase I, respectively. In addition, the Y361L mutant shows a marked sensitivity to a zinc chelator HPH-5 suggesting that the mutant has altered zinc coordination. These results demonstrate that a single amino acid change at a residue at the active site can lead to the generation of a mutant resistant to FTase inhibitors. Such a mutant may be valuable for the study of the effects of FTase inhibitors on tumor cells.

Design and in vivo analysis of potent non-thiol inhibitors of farnesyl protein transferase

Inhibitors of farnesyl protein transferase (FPTase) based upon a pseudotripeptide template are described that comprise an imidazole group substituted with a hydrophobic substituent. (1, 5)-Disubstitution of the imidazole group is shown to be the optimal array that leads to potent and selective inhibitors of FPTase. A variety of aryl and isoprenyl substituents are shown to afford effective inhibitors, and the mechanism by which these compounds inhibit FPTase has been investigated. The biochemical behavior of these compounds suggests that they bind to FPTase at the site usually occupied by the protein substrate. In experiments in cell culture, the methyl ester prodrugs of these inhibitors are cell permeant and potently inhibit the posttranslational modification of H-Ras protein. Additionally, these molecules revert the phenotype of ras transformed cells as evidenced by their ability to slow the growth of ras transformed cell lines in soft agar. One of the inhibitors, as its methyl prodrug, was evaluated in two in vivo models of tumor growth. The compound selectively inhibited the growth of tumors derived from H-ras transformed cells, in nude mice, and caused the regression of preexisting tumors in an H-ras transgenic animal model.

Synthesis and evaluation of hydroxyproline-derived isoprenyltransferase inhibitors

A series of peptidomimetics based on a hydroxyproline scaffold was prepared and evaluated for inhibition of farnesyltransferase and geranylgeranyltransferase I in both enzymatic and cell-based assays. A number of analogs were potent and selective inhibitors of FTase, while one compound (22) was nonselective in the enzymatic assays but eight-fold selective for inhibition of GGTase in the cellular assay (IC50 = 0.39 microM).

Zinc-catalyzed sulfur alkyation:insights from protein farnesyltransferase

Zinc metalloenzymes catalyze many important cellular reactions. Recently, the involvement of zinc in the catalysis of alkylation of sulfur groups has gained prominence. Current studies of the zinc metalloenzyme protein farnesyltransferase have shed light on its structure and catalytic mechanism, as well as the general mechanism of zinc-catalyzed sulfur alkylation.

Synthesis of Sulfur-Containing Olefinic Peptide Mimetic Farnesyl Transferase Inhibitors Using the Nozaki-Hiyama-Kishi Reaction and Cuprate S(N)2' Displacements

Syntheses of the potent sulfur-containing tetrapeptide mimetic farnesyl transferase inhibitors B956 (22) and B957 (23) are described. The two double bonds in 22 and 23 were constructed by application of iterative NHK and cuprate S(N)2' reactions. Normal syn NHK reaction and substrate-dependent syn and anti-S(N)2' diastereoselectivities accompanied by exclusive E-olefin selectivity were observed for the first NHK iteration (1 --> 4). In the second iteration, unexpected epimerization and a strong preference for syn diastereoselectivity was observed for the NHK reaction (5b --> 7a + 9a) while an unusual Z-olefin was observed for the S(N)2' reaction (7b --> 11). Deprotection conditions were optimized to ensure high purity and yield of the final aminothiol compounds.