Crystallographic analysis of CaaX prenyltransferases complexed with substrates defines rules of protein substrate selectivity.

Post-translational modifications are essential for the proper function of many proteins in the cell. The attachment of an isoprenoid lipid (a process termed prenylation) by protein farnesyltransferase (FTase) or geranylgeranyltransferase type I (GGTase-I) is essential for the function of many signal transduction proteins involved in growth, differentiation, and oncogenesis. FTase and GGTase-I (also called the CaaX prenyltransferases) recognize protein substrates with a C-terminal tetrapeptide recognition motif called the Ca1a2X box. These enzymes possess distinct but overlapping protein substrate specificity that is determined primarily by the sequence identity of the Ca1a2X motif. To determine how the identity of the Ca1a2X motif residues and sequence upstream of this motif affect substrate binding, we have solved crystal structures of FTase and GGTase-I complexed with a total of eight cognate and cross-reactive substrate peptides, including those derived from the C termini of the oncoproteins K-Ras4B, H-Ras and TC21. These structures suggest that all peptide substrates adopt a common binding mode in the FTase and GGTase-I active site. Unexpectedly, while the X residue of the Ca1a2X motif binds in the same location for all GGTase-I substrates, the X residue of FTase substrates can bind in one of two different sites. Together, these structures outline a series of rules that govern substrate peptide selectivity; these rules were utilized to classify known protein substrates of CaaX prenyltransferases and to generate a list of hypothetical substrates within the human genome.

Evolution of adverse changes in stored RBCs.

Recent studies have underscored questions about the balance of risk and benefit of RBC transfusion. A better understanding of the nature and timing of molecular and functional changes in stored RBCs may provide strategies to improve the balance of benefit and risk of RBC transfusion. We analyzed changes occurring during RBC storage focusing on RBC deformability, RBC-dependent vasoregulatory function, and S-nitrosohemoglobin (SNO-Hb), through which hemoglobin (Hb) O(2) desaturation is coupled to regional increases in blood flow in vivo (hypoxic vasodilation). Five hundred ml of blood from each of 15 healthy volunteers was processed into leukofiltered, additive solution 3-exposed RBCs and stored at 1-6 degrees C according to AABB standards. Blood was subjected to 26 assays at 0, 3, 8, 24 and 96 h, and at 1, 2, 3, 4, and 6 weeks. RBC SNO-Hb decreased rapidly (1.2 x 10(-4) at 3 h vs. 6.5 x 10(-4) (fresh) mol S-nitrosothiol (SNO)/mol Hb tetramer (P = 0.032, mercuric-displaced photolysis-chemiluminescence assay), and remained low over the 42-day period. The decline was corroborated by using the carbon monoxide-saturated copper-cysteine assay [3.0 x 10(-5) at 3 h vs. 9.0 x 10(-5) (fresh) mol SNO/mol Hb]. In parallel, vasodilation by stored RBCs was significantly depressed. RBC deformability assayed at a physiological shear stress decreased gradually over the 42-day period (P < 0.001). Time courses vary for several storage-induced defects that might account for recent observations linking blood transfusion with adverse outcomes. Of clinical concern is that SNO levels, and their physiological correlate, RBC-dependent vasodilation, become depressed soon after collection, suggesting that even "fresh" blood may have developed adverse biological characteristics.

Crystal structures of the anticancer clinical candidates R115777 (Tipifarnib) and BMS-214662 complexed with protein farnesyltransferase suggest a mechanism of FTI selectivity.

The search for new cancer therapeutics has identified protein farnesyltransferase (FTase) as a promising drug target. This enzyme attaches isoprenoid lipids to signal transduction proteins involved in growth and differentiation. The two FTase inhibitors (FTIs), R115777 (tipifarnib/Zarnestra) and BMS-214662, have undergone evaluation as cancer therapeutics in phase I and II clinical trials. R115777 has been evaluated in phase III clinical trials and shows indications for the treatment of blood and breast malignancies. Here we present crystal structures of R115777 and BMS-214662 complexed with mammalian FTase. These structures illustrate the molecular mechanism of inhibition and selectivity toward FTase over the related enzyme, protein geranylgeranyltransferase type I (GGTase-I). These results, combined with previous biochemical and structural analyses, identify features of FTase that could be exploited to modulate inhibitor potency and specificity and should aid in the continued development of FTIs as therapeutics for the treatment of cancer and parasitic infections.

Crystallographic analysis reveals that anticancer clinical candidate L-778,123 inhibits protein farnesyltransferase and geranylgeranyltransferase-I by different binding modes.

Many signal transduction proteins that control growth, differentiation, and transformation, including Ras GTPase family members, require the covalent attachment of a lipid group by protein farnesyltransferase (FTase) or protein geranylgeranyltransferase type-I (GGTase-I) for proper function and for the transforming activity of oncogenic mutants. FTase inhibitors are a new class of potential cancer therapeutics under evaluation in human clinical trials. Here, we present crystal structures of the clinical candidate L-778,123 complexed with mammalian FTase and complexed with the related GGTase-I enzyme. Although FTase and GGTase-I have very similar active sites, L-778,123 adopts different binding modes in the two enzymes; in FTase, L-778,123 is competitive with the protein substrate, whereas in GGTase-I, L-778,123 is competitive with the lipid substrate and inhibitor binding is synergized by tetrahedral anions. A comparison of these complexes reveals that small differences in protein structure can dramatically affect inhibitor binding and selectivity. These structures should facilitate the design of more specific inhibitors toward FTase or GGTase-I. Finally, the binding of a drug and anion together could be applicable for developing new classes of inhibitors.

Structure of mammalian protein geranylgeranyltransferase type-I.

Protein geranylgeranyltransferase type-I (GGTase-I), one of two CaaX prenyltransferases, is an essential enzyme in eukaryotes. GGTase-I catalyzes C-terminal lipidation of >100 proteins, including many GTP- binding regulatory proteins. We present the first structural information for mammalian GGTase-I, including a series of substrate and product complexes that delineate the path of the chemical reaction. These structures reveal that all protein prenyltransferases share a common reaction mechanism and identify specific residues that play a dominant role in determining prenyl group specificity. This hypothesis was confirmed by converting farnesyltransferase (15-C prenyl substrate) into GGTase-I (20-C prenyl substrate) with a single point mutation. GGTase-I discriminates against farnesyl diphosphate (FPP) at the product turnover step through the inability of a 15-C FPP to displace the 20-C prenyl-peptide product. Understanding these key features of specificity is expected to contribute to optimization of anti-cancer and anti-parasite drugs.