An eEF1A1 truncation encoded by PTI-1 exerts its oncogenic effect inside the nucleus

Background

The oncogene PTI-1 was originally isolated from a prostate cancer cell line by its capability to transform rat fibroblasts. The PTI-1 mRNA has a very eccentric structure as the 5′UTR is similar to prokaryotic 23S rRNA, while the major open reading frame and the 3′UTR corresponds to a part of the mRNA encoding human translation elongation factor eEF1A1. Thus, the largest open reading frame encodes a truncated version of eEF1A1 lacking the first 67 amino acids, while having three unique N-terminal amino acids. Previously, the UTRs were shown to be a prerequisite for the transforming capacity of the PTI-1 transcript. In this study, we have investigated the possible role of the UTRs in regulating protein expression and localization.

Methods

The protein expression profiles of a number of PTI-1 mRNA variants were studied in vitro and in vivo. Furthermore, the oncogenic potentials of the same PTI-1 mRNAs were determined by monitoring the capacities of stably transfected cells expressing these mRNAs to induce tumors in nude mice and form foci in cell culture. Finally, the cellular localizations of PTI-1 proteins expressed from these mRNAs were determined by fluorescence microscopy.

Results

The PTI-1 mRNA was found to give rise to multiple protein products that potentially originate from translation initiation at downstream, inframe AUGs within the major open reading frame. At least one of the truncated protein variants was also found to be oncogenic. However, the UTRs did not appear to influence the amount and identities of these truncated protein products. In contrast, our localization studies showed that the UTRs of the transcript promote a nuclear localization of the encoded protein(s).

Conclusions

Translation of the PTI-1 mRNA results in multiple protein products of which (a) truncated variant(s) may play a predominant role during cellular transformation. The PTI-1 UTRs did not seem to play a role in translation regulation, but appeared to contribute to a nuclear localization of the PTI-1 protein(s). This indicates that the PTI-1 protein(s) exert(s) its/their oncogenic function inside the nucleus.

The importance of P-loop and domain movements in EF-Tu for guanine nucleotide exchange

Elongation factor Ts (EF-Ts) is the guanine nucleotide exchange factor for elongation factor Tu (EF-Tu). An important feature of the nucleotide exchange is the structural rearrangement of EF-Tu in the EF-Tu.EF-Ts complex caused by insertion of Phe-81 of EF-Ts between His-84 and His-118 of EF-Tu. In this study, the contribution of His-118 to nucleotide release was studied by pre-steady state kinetic analysis of nucleotide exchange in EF-Tu mutants in which His-118 was replaced by Ala or Glu. Intrinsic as well as EF-Ts-catalyzed release of GDP/GTP was affected by the mutations, resulting in an approximately 10-fold faster spontaneous nucleotide release and a 10-50-fold slower EF-Ts-catalyzed nucleotide release. The effects are attributed to the interference of the mutations with the EF-Ts-induced movements of the P-loop of EF-Tu and changes at the domain 1/3 interface, leading to the release of the beta-phosphate group of GTP/GDP. The K(d) for GTP is increased by more than 40 times when His-118 is replaced with Glu, which may explain the inhibition by His-118 mutations of aminoacyl-tRNA binding to EF-Tu. The mutations had no effect on EF-Tu-dependent delivery of aminoacyl-tRNA to the ribosome.

Factor IXa inhibition by protease nexin-2/amyloid beta-protein precursor on phospholipid vesicles and cell membranes

Protease nexin-2/amyloid beta-protein precursor (PN-2/A beta PP) is a Kunitz-type protease inhibitor which has been shown to be a tight-binding inhibitor of enzymes, factors XIa and IXa (FIXa), suggesting a role for this protein in hemostasis. Since coagulant reactions are modulated on biologic surfaces, we investigated how 25:75 (mol/mol) phosphatidylserine/phosphatidylcholine vesicles (PSPC), thrombin-activated platelets, or umbilical vein endothelial cells influence inactivation of FIXa by PN-2/A beta PP. The Km of human or porcine FIXa activation of human factor X in the presence of PSPC, activated platelets, or endothelial cells in the absence or presence of thrombin-activated factor VIII (FVIIIa) was similar, (0.05-0.39 microM). The presence of FVIIIa increased the catalytic efficiency (kcat/Km ratio) of human and porcine factor IXa's activation of factor X 4952-406-fold, respectively. In the presence of PSPC, the Ki of human and porcine FIXa inhibition by PN-2/A beta PP was Ki = 1.9 x 10(-9) M and 5.8 x 10(-9) M, respectively. After the addition of FVIIIa to the reaction, the Ki for both human and porcine FIXa inhibition by PN-2/A beta PP on PSPC increased 13- and 4-fold to Ki = 2.5 x 10(-8) M and 2.4 x 10(-8) M, respectively. These Ki for inhibition of human FIXa on phospholipid vesicles by PN-2/A beta PP were similar when factor X activation was measured by chromogenic or activation peptide release assays. FVIIIa reduced the inhibition of FIXa by PN-2/A beta PP only in the presence of PSPC.(ABSTRACT TRUNCATED AT 250 WORDS)

Protease nexin-2/amyloid beta protein precursor. A tight-binding inhibitor of coagulation factor IXa

Protease nexin-2/amyloid beta protein precursor (PN-2/A beta PP) is an abundant, secreted platelet protein which is a potent inhibitor of coagulation Factor XIa. We examined other potential anticoagulant activities of PN-2/A beta PP. Purified Kunitz protease inhibitor domain of PN-2/A beta PP and PN-2/A beta PP itself were found to prolong the coagulation time of plasma and pure Factor IXa. The Kunitz protease inhibitor domain also inhibited the ability of Factor IXa to activate Factor X. PN-2/A beta PP inhibited Factor IXa with a Ki of 7.9 to 3.9 x 10(-11) M in the absence and presence of heparin, respectively. When the second-order rate constant of PN-2/A beta PP's inhibition of Factor IXa (2.7 x 10(8) M-1min-1) was compared to that of antithrombin III (3.8 x 10(6) M-1min-1), PN-2/A beta PP was at least a 71-fold more potent inhibitor of Factor IXa than antithrombin III. PN-2/A beta PP formed a complex with Factor IXa as detected by gel filtration and ELISA. The finding that PN-2/A beta PP is a potent inhibitor of Factor IXa could help to explain the spontaneous intracerebral hemorrhages seen in patients with hereditary cerebral hemorrhage with amyloidosis Dutch-type where there is an extensive accumulation of PN-2/A beta PP in their cerebral blood vessels.

Time and temperature dependence of granulocyte damage by leucotoxic supernatants from Pasteurella haemolytica A1

Bacterial exotoxins may contribute to the pathogenic potential of micro-organisms through interactions with cells of the host defence system as well as by directly damaging host tissue. The present studies were designed to explore mechanisms of interaction between bovine granulocytes and the leucotoxin produced by Pasteurella haemolytica, a major cause of bovine respiratory disease. Leucotoxin-containing supernatant from P. haemolytica A1 caused rapid cell death in isolated bovine granulocytes that was close to half-maximal by 5 min and nearly 90% complete after 30 min at 37 degrees C. Maintaining granulocytes at ice-water temperature markedly attenuated or prevented the toxic effect; furthermore, if exposed to supernatants at ice-water temperature and then washed, most cells remained viable even after rewarming to room temperature. However, even a very brief exposure (about 5 s) at 37 degrees C led to extensive cell death even after immediate cold dilution and washing. Granule enzymes such as arylsulphatase were released far more slowly than cytosol contents. Leucotoxin purified by column chromatography showed temperature dependence and divergence between cytosol and granule marker release similar to those observed with the crude supernatant preparation. These findings indicate that irreversible interaction between P. haemolytica leucotoxin and bovine granulocytes is initiated very rapidly at 37 degrees C but markedly impeded at low temperature, while granule enzyme release follows cytosol marker release over a much longer period. The results suggest either a requirement for target cell metabolic activity to initiate toxin effects or a temperature-dependent receptor conformation, with granule enzyme release following as a secondary consequence of granulocyte death.

Granulocyte plasma membrane damage by leukotoxic supernatant from Pasteurella haemolytica A1 and protection by immune serum

Bovine respiratory disease caused by Pasteurella haemolytica may be partially mediated by a leukotoxin secreted by the microorganism. We examined the effect of leukotoxic Pasteurella supernatants on leakage of the cytosol enzyme lactate dehydrogenase and the lysosomal enzyme arylsulfatase from bovine granulocytes. Lactate dehydrogenase release (94%) was much higher than arylsulfatase release (38%) over 30 minutes of incubation. The Pasteurella supernatants inhibited superoxide production by stimulated granulocytes at concentrations which also caused substantial cell death as measured by failure to exclude trypan blue. Both toxic effects were prevented by serum from aerosol-immunized calves, and protection appeared to be antibody-specific by comparison with fetal bovine serum or with serum absorbed against intact P. haemolytica. These findings suggest that the leukotoxin may selectively disrupt the granulocyte plasma membrane, and that antibody directed against a surface component of the microorganism is also capable of protecting against the leukotoxin effect.