Trypanosoma brucei prenylated-protein carboxyl methyltransferase prefers farnesylated substrates

Screening a Natural Product-Based Library against Kinetoplastid Parasites

Kinetoplastid parasites cause vector-borne parasitic diseases including leishmaniasis, human African trypanosomiasis (HAT) and Chagas disease. These Neglected Tropical Diseases (NTDs) impact on some of the world's lowest socioeconomic communities. Current treatments for these diseases cause severe toxicity and have limited efficacy, highlighting the need to identify new treatments. In this study, the Davis open access natural product-based library was screened against kinetoplastids (Leishmania donovani DD8, Trypanosoma brucei brucei and Trypanosoma cruzi) using phenotypic assays. The aim of this study was to identify hit compounds, with a focus on improved efficacy, selectivity and potential to target several kinetoplastid parasites. The IC50 values of the natural products were obtained for L. donovani DD8, T. b. brucei and T. cruzi in addition to cytotoxicity against the mammalian cell lines, HEK-293, 3T3 and THP-1 cell lines were determined to ascertain parasite selectivity. Thirty-one compounds were identified with IC50 values of ≤ 10 µM against the kinetoplastid parasites tested. Lissoclinotoxin E (1) was the only compound identified with activity across all three investigated parasites, exhibiting IC50 values < 5 µM. In this study, natural products with the potential to be new chemical starting points for drug discovery efforts for kinetoplastid diseases were identified.

In Vitro Trypanocidal Activity of Antibodies to Bacterially Expressed Trypanosoma brucei Tubulin

BACKGROUND

There are only four drugs for treating African trypanosomiasis, a devastating disease in sub-Saharan Africa. With slow discovery of better drugs, vaccination is viewed as the best method of control. We previously showed that antibodies to native Trypanosoma brucei brucei tubulin inhibit the growth of trypanosomes in culture. Here, we aimed to determine the effect of antibodies to bacterially expressed trypanosome tubulin on T. brucei brucei growth.

METHODS

T. brucei brucei alpha and beta tubulin genes were individually expressed in Escherichia coli under the tryptophan promoter. Monoclonal tubulin antibodies reacted specifically with the expressed tubulins with no cross-reaction with the opposite tubulin. Rabbits were immunized with 450µg each of the concentrated recombinant tubulin, and production of antibodies assessed by ELISA and Western blotting. The effect of polyclonal antibodies on trypanosome growth was determined by culturing bloodstream T. brucei brucei in up to 25% of antisera.

RESULTS

Low antisera dilutions (25%) from the immunized rabbits inhibited trypanosome growth. The most cytotoxic antisera were from one rabbit immunized with a mixture of both alpha and beta tubulins. However, the result was not reproduced in other rabbits and there was no apparent effect on growth at higher antisera dilutions.

CONCLUSION

Antibodies to bacterially expressed trypanosome tubulin are not effective at killing cultured bloodstream trypanosomes.

An essential farnesylated kinesin in Trypanosoma brucei

Kinesins are a family of motor proteins conserved throughout eukaryotes. In our present study we characterize a novel kinesin, Kinesin(CaaX), orthologs of which are only found in the kinetoplastids and not other eukaryotes. Kinesin(CaaX) has the CVIM amino acids at the C-terminus, and CVIM was previously shown to be an ideal signal for protein farnesylation in T. brucei. In this study we show Kinesin(CaaX) is farnesylated using radiolabeling studies and that farnesylation is dependent on the CVIM motif. Using RNA interference, we show Kinesin(CaaX) is essential for T. brucei proliferation. Additionally RNAi Kinesin(CaaX) depleted T. brucei are 4 fold more sensitive to the protein farneysltransferase (PFT) inhibitor LN-59, suggesting that Kinesin(CaaX) is a target of PFT inhibitors' action to block proliferation of T. brucei. Using tetracycline-induced exogenous tagged Kinesin(CaaX) and Kinesin(CVIMdeletion) (non-farnesylated Kinesin) expression lines in T. brucei, we demonstrate Kinesin(CaaX) is farnesylated in T. brucei cells and this farnesylation has functional effects. In cells expressing a CaaX-deleted version of Kinesin, the localization is more diffuse which suggests correct localization depends on farnesylation. Through our investigation of cell cycle, nucleus and kinetoplast quantitation and immunofluorescence assays an important role is suggested for Kinesin(CaaX) in the separation of nuclei and kinetoplasts during and after they have been replicated. Taken together, our work suggests Kinesin(CaaX) is a target of PFT inhibition of T. brucei cell proliferation and Kinesin(CaaX) functions through both the motor and farnesyl groups.

The polybasic region of Rho GTPases defines the cleavage by Yersinia enterocolitica outer protein T (YopT)

Pathogenic Yersinia strains evade the innate immune responses of the host by producing effector proteins ( Yersinia outer proteins [Yops]), which are directly injected into mammalian cells by a type III secretion system (TTSS). One of these effector proteins (YopT) disrupts the actin cytoskeleton of the host cell resulting in cell rounding. YopT is a cysteine protease that cleaves Rho proteins directly upstream of the post-translationally modified cysteine. Thereby, it releases the GTPases from the membrane leading to inactivation. Small GTPases are modified by isoprenylation of the cysteine of the CAAX box, cleavage of the -AAX tripeptide, and methylation of the cysteine. We have shown that isoprenylation and the endoproteolytic cleavage of the tripeptide of Rho GTPases are essential for YopT-induced cleavage, whereas carboxyl methylation is not required. In the present study, we post-translationally modified RhoA, Rac, Cdc42, and several mutants in vitro and characterized the YopT-induced cleavage with recombinant YopT. We show that farnesylated RhoA is a preferred substrate of YopT compared with the geranylgeranylated GTPase. Geranylgeranylated RhoA, however, is the preferred substrate for YopT-catalyzed cleavage with a threefold faster turnover rate over Rac and Cdc42. Moreover, our data indicate that the composition of the polybasic region of the GTPases defines the specificity and efficiency of the YopT-induced cleavage, and that a space between the polybasic stretch of amino acids at the C terminus and the CAAX box enhances the turnover rate of YopT-catalyzed cleavage.

Thematic review series: lipid posttranslational modifications. Fighting parasitic disease by blocking protein farnesylation

Protein farnesylation is a form of posttranslational modification that occurs in most, if not all, eukaryotic cells. Inhibitors of protein farnesyltransferase (PFTIs) have been developed as anticancer chemotherapeutic agents. Using the knowledge gained from the development of PFTIs for the treatment of cancer, researchers are currently investigating the use of PFTIs for the treatment of eukaryotic pathogens. This "piggy-back" approach not only accelerates the development of a chemotherapeutic agent for protozoan pathogens but is also a means of mitigating the costs associated with de novo drug design. PFTIs have already been shown to be efficacious in the treatment of eukaryotic pathogens in animal models, including both Trypanosoma brucei, the causative agent of African sleeping sickness, and Plasmodium falciparum, one of the causative agents of malaria. Here, current evidence and progress are summarized that support the targeting of protein farnesyltransferase for the treatment of parasitic diseases.

Purification, functional reconstitution, and characterization of the Saccharomyces cerevisiae isoprenylcysteine carboxylmethyltransferase Ste14p

Numerous proteins, including Ras, contain a C-terminal CAAX motif that directs a series of three sequential post-translational modifications: isoprenylation of the cysteine residue, endoproteolysis of the three terminal amino acids and alpha-carboxyl methylesterification of the isoprenylated cysteine. This study focuses on the isoprenylcysteine carboxylmethyltransferase (Icmt) enzyme from Saccharomyces cerevisiae, Ste14p, the founding member of a homologous family of endoplasmic reticulum membrane proteins present in all eukaryotes. Ste14p, like all Icmts, has multiple membrane spanning domains, presenting a significant challenge to its purification in an active form. Here, we have detergent-solubilized, purified, and reconstituted enzymatically active His-tagged Ste14p from S. cerevisiae, thus providing conclusive proof that Ste14p is the sole component necessary for the carboxylmethylation of isoprenylated substrates. Among the extensive panel of detergents that was screened, optimal solubilization and retention of Ste14p activity occurred with n-dodecyl-beta-d-maltoside. The activity of Ste14p could be further optimized upon reconstitution into liposomes. Our expression and purification schemes generate milligram quantities of pure and active Ste14p, which is highly stable under many conditions. Using pure reconstituted Ste14p, we demonstrate quantitatively that Ste14p does not have a preference for the farnesyl or geranylgeranyl moieties in the model substrates N-acetyl-S-farnesyl-l-cysteine (AFC) and N-acetyl-S-geranylgeranyl-l-cysteine (AGGC) in vitro. In addition to catalyzing methylation of AFC, we also show that purified Ste14p methylates a known in vivo substrate, Ras2p. Evidence that metals ions are required for activity of Ste14p is also presented. These results pave the way for further characterization of pure Ste14p, as well as determination of its three-dimensional structure.